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1 University of Colorado, Boulder CU Scholar Chemistry & Biochemistry Graduate Theses & Dissertations Chemistry & Biochemistry Spring Ferroelectric and Antiferroelectric Odd-Even Behavior in a Tricarbosilane-Terminated Liquid Crystal Homologous Series and An Electric-Field- Responsive Discotic Columnar Liquid Crystal Nan Hu University of Colorado Boulder, hunanmck@gmail.com Follow this and additional works at: Part of the Chemistry Commons Recommended Citation Hu, Nan, "Ferroelectric and Antiferroelectric Odd-Even Behavior in a Tricarbosilane-Terminated Liquid Crystal Homologous Series and An Electric-Field-Responsive Discotic Columnar Liquid Crystal" (2013). Chemistry & Biochemistry Graduate Theses & Dissertations This Dissertation is brought to you for free and open access by Chemistry & Biochemistry at CU Scholar. It has been accepted for inclusion in Chemistry & Biochemistry Graduate Theses & Dissertations by an authorized administrator of CU Scholar. For more information, please contact cuscholaradmin@colorado.edu.

2 Ferroelectric and Antiferroelectric Odd- Even Behavior in a Tricarbosilane- Terminated Liquid Crystal Homologous Series and An Electric-Field-Responsive Discotic Columnar Liquid Crystal Nan Hu B.S., Anhui Normal University, 2006 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment Of the requirement for the degree of Doctor of Philosophy Department of Chemistry and Biochemistry 2013

3 This thesis entitled: Ferroelectric and Antiferroelectric Odd-Even Behavior in a Tricarbosilane-Terminated Liquid Crystal Homologous Series and An Electric-Field-Responsive Discotic Columnar Liquid Crystal Written by Nan Hu Had been approved for the Department of Chemistry and Biochemistry Professor David M. Walba Professor Douglas L. Gin Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline.

4 Hu, Nan (Ph.D., Chemistry) Ferroelectric and Antiferroelectric Odd-Even Behavior in a Tricarbosilane-Terminated Liquid Crystal Homologous Series and An Electric-Field-Responsive Discotic Columnar Liquid Crystal Thesis directed by Professor David M. Walba Two separate projects on thermotropic liquid crystals are described: an odd-even behavior giving ferroelectric or antiferroelectric phases in a tricarbosilane-terminated liquid crystal homologous series, and an electric-field-responsive discotic columnar liquid-crystal formed from an hexa-peri-hexabenzocoronene/oligothiophene hybrid. A new class of polyphilic mesogens composed of a tolanphenyl carboxylate core, a chiral alkoxy tail, and a tricarbosilane terminated alkoxy tail were synthesized and shown to selfassemble into nanophase-segregated smectic liquid-crystalline (LC) phases. It was found that the number of carbons in the alkoxy spacer between the tricarbosilane and the core determined the phase observed with perfect fidelity: homologues with odd-carbon spacers exhibit antiferroelectric phases, while those with even-carbon spacers exhibit ferroelectric phases. Even more interestingly, homologues with an odd carbon number show the rare and useful chiral orthoconic antiferroelectric SmC A * phase, in which the optic axis tilt alternates layer to layer between +45 and -45, making the molecular directors in adjacent layers orthogonal. The evencarbon homologues exhibit the SmC* phase with a 45 tilt angle. These findings demonstrate that the supramolecular self-assembly of polyphilic mesogens may in some cases be controlled by the length of the hydrocarbon spacers. A novel hexa-peri-hexabenzocoronene (HBC) derivative with six covalently tethered alkyl-trizaole-quadra-3-hexylthiophene units was synthesized via click chemistry and shown to iii

5 self-assemble into a hexagonal columnar liquid-crystal (LC) phase. Compared to other HBCbased LC materials, this mesogen shows unprecedented responses to applied electric fields in cells with thickness of several microns, resulting in uniform homeotropic or parallel alignment depending upon the electrode structure. Furthermore, the columnar orientation, once developed by an applied electric field, can be maintained even after removal of the electric field unless the material is heated above its clearing temperature. iv

6 Table of Contents Abstract.....iii Table of Contents..iv List of Tables....vi List of Figures. vii List of Schemes.... viii 1 Introduction Liquid Crystals Calamitic Liquid Crystals Ferroelectricity and Antiferroelectricity in Liquid Crystals Symmetry and Chirality in Liquid Crystal Phases Ferroelectric LCs and Surface Stabilization Antiferroelectric Liquid Crystals Bent-core Liquid Crystals Bent-core Liquid Crystals B 2 Phase Discotic Liquid Crystals Experimental Techniques Odd-Even Behavior of Ferroelectricity and Antiferroelectricity in Tricarbosilane- Terminated Liquid Crystals Introduction Goals and Prior Work Design and Synthesis Results and Discussion Mesomorphic Properties of 1(n) Homologues Nanophase segregated silane sublayer Experimental Procedures An Electric-Field-Responsive Discotic Liquid-Crystalline Hexa-perihexabenzocoronene/Oligothiophene Hybrid Introduction Goals Design and Synthesis Results and Discussion v

7 3.5 Experimental Procedures Bibliography vi

8 List of Figures Figure 1.1 Three common molecular shapes for thermotropic mesogens: discotic (disk-shaped), calamitic (rod-shaped), and bent-core (chevron-shaped) Figure 1.2 Three common calamitic LC phases are nematic, smectic A, and smectic C, listed in order of descending temperature. The individual molecules have an average alignment along the director (n), and the orientation of the layers in lamellar phases is typically described by the layer normal (z)... 4 Figure 1.3 Helical super-structure in the SmC* phase... 8 Figure 1.4 Orientations of a surface-stabilized AFLC in (a) positive, (b) zero, and (c) negative applied fields. Cell substrates are parallel to the paper, and the smectic layer normal is along ŷ Figure 1.5 The chemical structure of the classic bent-core mesogen series P-n-O-PIMB (left) and symbolic representations of the bent-core molecules with the shape of an arrow (center). The vertical white arrow in the center figure represents the molecular director (n), and the black arrow represents the two-fold symmetry axis (the arrow of the bow and arrow) (b) Figure 1.6 In the SmCP phase, three planes, the tilt plane (xz), the layer plane (xy) and polar plane (yz) are defined as shown in (a). Four diastereomeric members of the B 2 family of subphases (SmCP phase) are showed in (b). In the SmC A P A and SmC S P F phases, the layer chirality (indicated by blue and red) is identical in adjacent layers. In the SmC A P F and SmC S P A phases, the layer chirality alternates from layer to layer, providing net achiral macroscopic racemates.. 14 Figure 1.7 Different types of nematic phases formed by discotic mesogens: a) Illustration of a conventional nematic phase of calamitic mesogens; b) Discotic nematic phase N D ; and c) Columnar nematic mesophases (N C ) induced by the charge-transfer interactions between a diskshaped donor (black) and an electron acceptor (grey). The respective building blocks of the phase (disk-shaped molecules or columns) show long-range orientational order with no longrange positional order, so that all these nematic phases have the same symmetry Figure 1.8 Plane views of the 2D lattices in hexagonal (a), rectangular (b-d), and oblique (e) columnar mesophases. Point-group symmetries in parentheses are according to the international system Figure 1.9 The presence of an anisotropic transparent birefringent sample between two crossed polarizing plates will allow light entering through the first plate to exit the second plate Figure 1.10 An example of a DSC curve, showing two peaks caused by first-order phase transitions Figure 2.1 Molecular structures and composition of W107, the first reported OAFLC mixture. 27 Figure 2.2 a) Illustration of the local layer structure of the SmC*; and b) The SmC A * phases of the prototype antiferroelectric MHPOBC (molecular structure given in c). The layer interfaces (horizontal lines) in the uniformly tilted SmC structure are denoted as synclinic, while the layer interfaces in the alternately tilted SmC A are denoted as anticlinic. The chiral methylheptyloxycarbonyl grouping is thought to be the key chemical structure feature leading to the rare anticlinic phases such as the SmC A vii

9 Figure 2.3 Schematic illustration of the organization process taking place during solidification of alkyl-substituted discotics from the isotropic phase Figure 2.4 (a) Molecular structure of W586 and its phase sequence on heating; (b) The layer spacing obtained from X-ray is about 61 Å, much larger than the calculated extended molecular length of 52 Å, implying a structure in which adjacent molecules have antiparallel packing. The silane termination suppresses out-of-layer fluctuations to promote ferroelectric coupling (anticlinic ordering) of adjacent layer; (c) Reducing the number of tails from two to one per molecule creates more space for the tails, promoting orthogonal phases. Out-of-layer fluctuations (ellipse) enable penetration of tails into the adjacent layers favoring synclinic tail tilt at the layer interfaces and thus macroscopic antiferroelectric (SmAP A ) order shown by bent-core structures without a carbosilane or similar group (comparison). Introducing carbosilane into the tail suppresses out-of-layer fluctuations, favoring anticlinic tail orientation and thus the SmAP F structure observed from W Figure 2.5 Molecular structures of the compounds under investigation Figure 2.6 DSC traces of 1(n) on second heating/cooling at a scan rate of 2 C min Figure 2.7 (a) For the odd number homologs, a dark state can be observed off the electrode in surface-stabilized planar aligned LC cells (less than 3 µm thick); In a 3.5 µm-thick cell, some subtle textures were visible, indicative of helix formation. (b) The photomicrograph showing the switching behavior from SmC A * to SmC* under the application of an electric field. (c) The ferroelectric state with the expected bright birefringence can be observed Figure 2.8 The photomicrograph showing domain-wall mediated behavior under application of an electric field +0.5 V/µm to a 3.1- m-thick cell of the homologues with the even number of alkyl segments W651_1(10), W696_1(8), W641_1(6) Figure 2.9 Molecular orientations in an odd number and even number of smectic layers in SmC* and SmC A * phases. Green marks indicate transverse polarization (the net spontaneous polarization normal to the tilt plane) and yellow arrows indicate longitudinal polarization (the net polarization parallel to the tilt plane). (a) As for SmC* films, both of odd number and even number of smectic layers have a net transverse polarization. (b) As for SmC A * films, films with an even number of smectic layers have a net longitudinal polarization while films with an odd number of layers have a net transverse polarization Figure 2.10 Regions with odd (o) and even (e) number of layers of freely suspended W617_1(11) film at T=70 o C, analyzer is decrossed by +3 o and an applied square wave electric field of E=28.5 V/mm with a frequency of 0.2Hz Figure 2.11 Regions of varying layer thicknesses at T= 40 o C with an applied square wave electric field of E=0.057 V/mm with a frequency of 0.2Hz. The number of layers of each region is labeled on the images. a) Analyzer is decrossed +20 o (away from the electric field). Regions appear bright since molecules have transverse polarization. b) Analyzer is decrossed -20 o (towards the electric field) Figure 2.12 A second harmonic of the layer peak in the W617_1(11) and W651_1(10) Figure 2.13 X-ray measurements of layer spacings of all mesogens in this series, denoted by d, as function of T Figure 2.14 Polarization of all mesogens in this series, as a function of T on cooling viii

10 Figure 2.15 Proposed of the organization of 1(n) when n is even number (a) and when n is odd number (b) in the triply segregated smectic phase Figure 2.16 One conformer of 1(11) (a) and 1(10) (b) for illustration of the models in Figure Figure 2.17 The molecular structures and bent-core structures of W586 homologues Figure 3.1 Typical length scales encountered in organic electronics and control of order achievable with LC semiconductors Figure 3.2 Schematic illustration of calamitic (left) and discotic (right) semiconductors Figure 3.3 Selected examples of widely studied discotic HBC derivatives Figure 3.4 Schematic illustration of two types of electronic devices and their desired supramolecular arrangements on surfaces with edge-on orientation of the molecules for FETs versus face-on arrangement for photovoltaic devices Figure 3.5 Schematic illustration of the deposition of the amphiphilic HBC molecules onto a substate with the edge-on orientation using the LB technique. 7b Figure 3.6 Schematic illustration of zone-casting HBC-C12. 6b Figure 3.7 Schematic representation of the columnar HBC stacking with uniaxially parallel alignment. 6b Figure 3.8 Chemical structures of HBC derivatives with spontaneous homeotropic alignment. 79 Figure 3.9 Structures and phase diagrams of COL LCs that are responsive to electric fields and their columnar orientation is maintained even after the E field is switched off. Phase-transition temperatures are in C. Symbols Cr, Col h, and Iso denote crystalline, hexagonal columnar, and isotropic phases, respectively. Values in parentheses below the symbol Col are intercolumnar distances in nm, and the subscripted values are the observed temperatures in C. 29e Figure 3.10 Retro synthesis of a HBC/ quaterthiophene hybrid HBC-4Th Figure 3.11 Molecular structure of mesogen Figure H NMR (400 MHz) of 1 in CDCl 3 at 25 C Figure C NMR (101 MHz) of 1 in CDCl 3 at 25 C Figure C NMR (101 MHz) of 1 above 100 ppm in CDCl 3 at 25 C Figure 3.15 Normalized UV-vis measurements in chloroform for 1, the oligothiophene arm, and the HBC core Figure 3.16 DSC traces of 1 on second heating/cooling at a scan rate of 2 C min Figure 3.17 (a) Texture observed for the Col h phase of mesogen 1 at 80 C (Red scale bar represents 100 µm). (b) FFTEM image of a Pt-C replica showing the topography of mesogen 1 quenched from the Col h phase (at T = 80 C) and fractured in the bulk, magnified in (c). (d) Simplified model of the structure observed in (c). (e) Synchrotron small angle scattering observed for a powder sample of 1, and (f) an azimuthal scan of the wide angle scattering obtained from a 2D diffraction pattern for a powder sample of 1 at 84 C. The original 2D wide angle data is given in (g) ix

11 Figure 3.18 Mesogen 1 was sandwiched between two sheets of ITO patterned glass separated by 4μm at 80 C. A) Zero field; B) An AC E-field (10 Hz, 28 V/μm) was applied vertical to the sample under crossed polarizers Figure 3.19 A plot of the switching voltage threshold for driving the reorientation of a parallel sample to homeotropic arrangement as a function of temperature Figure 3.20 (a and b) POM images (red scale bar represents 100 µm) obtained for LC cells of 1 at 80 C between crossed polarizer and analyzer. Mesogen 1 was filled by capillary action into a sandwich-type glass LC cell with patterned ITO electrodes (the cell gap is 4µm). (a) An AC square wave electric field (2.0 Hz, ±28V/µm) was applied to the sample at 80 C. The photomicrograph shows the edge of the electrode area. Domains of unoriented parallel alignment can be seen on the left, off the electrodes with no applied field, and on the right homeotropic alignment over the ITO-coated area. (b) A 2.0 Hz AC square wave in plane field (±15 V/ m) was applied to the sample to provide clean parallel alignment. One substrate of the cell was patterned with ITO electrodes with a 10 µm pitch, to provide the in-plane field in stripes between the electrodes (all adjacent electrodes are driven with opposite sign of E. The field was applied while cooling the sample from Iso to Col h. The insert shows a detail of the aligned sample, showing bright stripes of parallel-aligned Col h phase. (c and d) 2D-SAXS patterns obtained from an area off the ITO electrodes (c) and over the ITO electrodes (d) from an LC cell (cell gap 6.4 µm) fabricated using thin (80 µm) glass plates each coated with an ITO square. The sample was heated to 80 C and driven by a vertical E field (±20 V/ m) 10 days before the XRD data was collected. The homeotropically aligned sample (d) shows point-like reflections arranged in a hexagonal lattice. The lattice is indicated by lines added to the image after data collection to guide the eye. (e and f) Schematic illustration of the different types of supramolecular arrangements in (c) and (d) with the incident X-ray beam (indicated by red arrow) Figure 3.21 A square 2.0 Hz E field (15 V/ m) was site-selectively applied to the sample 1 with a 10 m gap of ITO electrodes from a horizontal direction relative to the glass substrates while it was cooling from Iso to Col h. Compared with Figure 3.20b, the magnified (insert) indicates much darker stripes, which were driven by a horizontal E-field (the direction is shown in double red arrow) between two ITO electrodes with a 10 m gap and oriented parallel to the analyzer. 99 Figure 3.22 Current profile of an oriented film (thickness, 4 μm) of 1 at 80 C upon application of a triangular-shaped AC field (10.0 Hz), indicates undetectable polarization reversal current x

12 List of Schemes Scheme 2.1 Synthesis of a series of tricarbosilane-terminated compounds 1(n), W617_1(11), W651_1(10), W650_1(9), W696_1(8), W677_1(7), W641_1(6), W676_1(5) Scheme 3.1 Synthesis of ethynyl-substituted quaterthiophene Scheme 3.2 Tentative synthesis of HBC-6QTh Scheme 3.3 Synthesis of HBC-hexa-3-hexyl-quadrathiophene xi

13 List of Tables Table point groups and electric properties allowed in these point groups Table 2.1 [a] Structure of the new tricarbosilane-terminated 1(n), phase sequence and transition temperatures, and clearing enthalpy, from a combination of DSC, XRD, POM, EO and DRLM data; [b] layer spacing at 55 C xii

14 1 Introduction 1.1 Liquid Crystals Liquid Crystals (LCs) are molecules that aggregate into an intermediate state between ordered crystals and disordered isotropic liquids. A substance in an LC state is strongly anisotropic in some of its properties and also shows a certain degree of fluidity, which in some cases might be comparable to that of an ordinary liquid. 1 LC phases are called mesophases, and the LC substances are termed mesogens. The first LC was discovered by Friedrich Reinitzer in Since then, thousands of organic compounds have been known to form LC phases. A key requirement for mesogens is that the molecule must be highly anisotropic in molecular shape, like a rod, a chevron, or a disk. The LC behavior of the materials described in this thesis results from immiscibility of different fragments of the molecules. Typical LC molecules, with the important exception of nematics, will contain two or more groups with limited compatibility, so that each group prefers to interact with the corresponding groups on adjacent molecules rather than with the incompatible groups. Depending on the mesogen structure, an LC might exhibit one or more mesophases before a transition into the isotropic liquid. Transitions to these intermediate phases can be triggered by a purely thermal process a kind of temperature-driven supramolecular isomerization (thermotropic phases) or the concentration of LC molecules in a solvent (lyotropic phases). Lyotropic LCs include two or more components that show LC properties in certain concentration ranges. In the lyotropic phases, solvent molecules fill the space around supramolecular aggregates of mesogens to contribute fluidity to the system. Lyotropic LC mesogens usually consist of one or more long, nonpolar alkyl tails that are hydrophobic, and an 1

15 ionic moiety often termed the head group, that is hydrophilic. Such lyotropic systems are formed through nanophase segregation of the two immiscible components in, for example, water as solvent. Varying concentrations of mesogens changes the self-assembly to provide different phases, such as cubic, hexagonal, lamellar, etc. Thermotropic LCs can be pure substances or miscible mixtures of mesogens exhibiting one or more anisotropic liquid crystal phases. At the top of the LC phase temperature range (the clearing point), the entropy of thermal motions dominates enthalpic factors favoring the selfassembly, pushing the material into a conventional isotropic liquid phase. If an LC phase is observed in a similar temperature range during both heating and cooling, the phase is termed enantiotropic, and exists as the thermodynamic minimum energy configuration in the phase temperature range. If the LC phase is seen on cooling, but not on heating from a lower temperature phase, then the phase is not the thermodynamic minimum, but rather a metastable state. Such phases are termed monotropic. Thermotropic liquid crystals are also classified by mesogen shapes, including calamitic, bent-core, and discotic mesogens. As shown in Figure 1.1, calamitic (rod-like) mesogens can form the nematic phase (N) or lamellar phases (smectics); bent-core (chevron-shaped) mesogens can form nematic, smectic and/or columnar phases; and discotic (disc-shaped) mesogens can exhibit nematic and/or columnar phases. Despite significant differences in their chemical compositions, mesogens typically share common structural features. For example, they usually possess a combination of rigid aromatic cores, flexible tails, and the highly anisotropic molecular shape. The work described herein will focus on LC phases and behaviors of calamitic and discotic mesogens. 2

16 Figure 1.1 Three common molecular shapes for thermotropic mesogens: discotic (disk-shaped), calamitic (rod-shaped), and bent-core (chevron-shaped). 1.2 Calamitic Liquid Crystals A calamitic mesogen is typically a rod-like molecule composed of two or more aromatic rings (the core) and aliphatic tails. Most calamitic mesogens exhibit more than one LC phases. There are three common and well-studied phases for calamitic mesogens: Nematic (N), Smectic A (SmA), and Smectic C (SmC), as indicated in Figure 1.2. Interestingly, nematic mesogens can be completely aliphatic, with features such as cyclohexane rings in the core. However, to our knowledge all known smectic mesogens contain at least one aromatic ring in the core. 3

17 Figure 1.2 Three common calamitic LC phases are nematic, smectic A, and smectic C, listed in order of descending temperature. The individual molecules have an average alignment along the director (n), and the orientation of the layers in lamellar phases is typically described by the layer normal (z). The nematic phase is the least ordered among the three phases, and has some degree of long-range orientational order, but no long-range positional order. Thus, the nematic phase differs from the corresponding isotropic phase in that the molecules are spontaneously oriented with their long axes approximately parallel. The molecules are free to flow, and the centers of mass positions are randomly distributed as in a liquid, but the bulk still maintains long-range 4

18 orientational order. Most, or perhaps all, nematic phases are uniaxial: they have one preferred axis (typically defined by the long axis of the ordered mesogenic rods, or the molecular director n), with the two orthogonal directions normal to the long axis being equivalent. In the nematic phase, the local ensemble average molecular long axis (the unique macroscopic axis, also denoted n) is parallel to the ensemble average of the molecular director, the latter possessing some finite degree of orientational disorder described by an order parameter. Mesophases owe their fluidity to the ease with which the molecules slide past one another while still retaining their macroscopic orientational order. The macroscopic director of a sample of nematic phase can be easily aligned by an applied electric or magnetic field. The uniformly aligned mesogens are optically uniaxial and strongly birefringent, this being a key feature rendering nematic LCs useful in devices such as liquid crystal displays (LCDs). Upon further cooling, the mesogens will often spontaneously isomerize into a new phase possessing well-defined layers, generating a smectic (Sm) phase. Sm phases have orientational order as well as positional order in one dimension. In the SmA phase, the molecules are oriented along the layer normal (z) with their centers irregularly spaced in a liquid-like fashion. Since the director (n) is defined by the long axis of the molecules, it is parallel to z in the SmA phase. In the SmC phase, molecules are in general more ordered (exhibit a higher order parameter), with their long axes tilted away from the layer normal z by an angle denoted the tilt angle. The tilt angle is typically temperature dependent, and increases with decreasing temperature until it saturates at a value characteristic of the particular SmC material. The tilt plane is defined as the plane including the layer normal and the macroscopic director. In this thesis, work related to calamitic LCs is described in Chapter 2. 5

19 1.3 Ferroelectricity and Antiferroelectricity in Liquid Crystals Symmetry and Chirality in Liquid Crystal Phases Materials exhibiting LC phases are usually composed of organic molecules with rather complex chemical structures. The symmetry of LC phases is generally described using point groups in an ad-hoc way. Every point in uniform supramolecular domain of system is examined for symmetry elements, (rotation or rotation/reflection axes), and the point group of the phase is simply the group of all of these elements. For crystals, as shown in Table 1.1, there are thirtytwo classes of crystal symmetries, or crystallographic point groups. Twenty-one of these lack inversion symmetry, and among these, twenty are piezoelectric (the point group O being the exception). Of these twenty classes piezoelectric crystal structures, ten are pyroelectric. Piezoelectric crystals exhibit a dielectric polarization under applied electric fields. But pyroelectric crystals possess a temperature dependent spontaneous polarization even in the absence of an electric field. Pyroelectric crystals must necessarily possess polar symmetry. Table 1.1 Thirty-two point groups and electric properties allowed in these point groups. 6

20 The point groups of LC phases differ in some ways from those of crystals, because LC phases are dynamic, and their symmetry is an ensemble or time average property. In LC phases, each molecule exists in one conformation on a very short time scale, and a great many of these conformations are in dynamic equilibrium in the phase. In discussing the symmetry of the phase only time-averaged conformations of molecules are considered, so that molecules can be regarded as simple rod, chevron (bent-core) or discotic shapes. The point group symmetry of both the nematic and SmA phases of cylindrical rod-shaped mesogens is D h, (an enumeration of the space groups of LCs has not been accomplished to date). Similarly, the symmetry of the SmC phase is C 2h. None of these symmetries (or phases) is pyroelectric. Liquid crystal phases with polar symmetry, being similar to pyroelectric crystals, are often termed ferroelectric. To be ferroelectric, it is necessary for the LC phase to have sufficiently low symmetry (polar symmetry), in most known cases either C n or C nv. R.B Meyer first realized that ferroelectric behavior can be achieved by introduction of chirality into the SmC phase. 2 As mentioned above, the point group symmetry of SmC phase is C 2h. Of course, the reflection symmetry is necessarily lost if the molecules are chiral, and the sample is non-racemic (SmC* phase), leading to a fluid phase with C 2 symmetry, which is polar. Thus, aligned domains of SmC* phase possesses a spontaneous macroscopic electric polarization in the absence of applied fields. In a glass liquid crystal cell, the SmC* phase can give a dielectric response identical to a true solid state ferroelectric thus the term ferroelectric liquid crystal is commonly applied to SmC* phases (vide infra). Polar symmetry (and resulting properties) can be found in other system as well. For example, for tilted discotic columnar phases, the introduction of chirality on the side chains works in a similar way as in the tilted chiral smectic phases, lowering the symmetry of the 7

21 structure to C 2 and allowing spontaneous electric polarization. 3,4 More recently, it has been shown that polar symmetry can be obtained in LCs without the requirement for molecular chirality. This occurs when chevron shaped (bent-core) or cone-shaped (conical) molecules fill space to give polar symmetry, yet remain fluid. 5,6,7 In addition, T. Aida and coworkers recently reported that a ferroelectric columnar LC featuring confined polar groups within an umbrella architecture Ferroelectric LCs and Surface Stabilization The study of ferroelectric liquid crystals (FLCs), a class of materials first discovered just thirty-nine years ago, has grown rapidly since the invention of the surface-stabilized ferroelectric liquid crystal (SSFLC) device by N. A. Clark and S. T. Lagerwall in Ferroelectricity is a property of certain materials that possess a spontaneous electric polarization that can be switched between two bistable states with antiparallel polarization, by application of an external electric field. 10 As mentioned above, introduction of chirality to the SmC phase lowers the molecular symmetry from C 2h to C 2, and allows the occurrence of a spontaneous polarization normal to the tilt plane. However, in bulk, molecular chirality forces the tilt direction to precess slightly from layer to layer forming a helical structure (sometimes termed helielectric) (Figure 1.3). This helix causes the spontaneous polarization to average to zero. To observe the spontaneous polarization in bulk, the helix must be unwound. Figure 1.3 Helical super-structure in the SmC* phase. 8

22 N. A. Clark and S. T. Lagerwall invented the surface stabilized ferroelectric liquid crystal light valve (SSFLC), in which the SmC* helix is suppressed by surface constraints. 9 In the SSFLC geometry, smectic layers align more or less perpendicular to the substrates, and the director prefers to orient more or less parallel to the substrates. The helix is spontaneously unwound when the cell thickness is on the order of the helical pitch. Two bistable surfacestabilized domains with antiparallel orientation of the ferroelectric polarization are observed, and the orientation of the polarization can switched by applied electric fields above a characteristic threshold value Antiferroelectric Liquid Crystals In the ferroelectric SmC* phase, if the helix is unwound the molecules tilt in the same direction from layer to layer with the same tilt angle, thus the spontaneous polarization orients almost uniformly in an SSFLC cell. Antiferroelectric liquid crystals (AFLCs) are chiral tilted smectics existing in a phase denoted SmC * A, in which the director in adjacent layers tilts in opposite directions relative to the layer normal (anticlinic order). The anticlinic molecular arrangement results in an antipolar ordering where the direction of local polarization alternates between adjacent layers, averaging to zero in layer pairs. This antiferroelectric phase is regarded as the third stable state (AFLC phase) by A. D. L. Chandani et al. 11, 12 When an external electric field above a characteristic threshold value is applied, the spontaneous polarization orients uniformly along the field to produce field-induced SmC* states, as indicated in Figure

23 Figure 1.4 Orientations of a surface-stabilized AFLC in (a) positive, (b) zero, and (c) negative applied fields. Cell substrates are parallel to the paper, and the smectic layer normal is along ŷ. 13 Research on AFLCs for display applications started in 1988, even without knowledge of molecular orientations or director structures. 11 At that time, ferroelectric liquid crystal displays were very promising because of their many advantages for passive matrix addressing, including fast response speed (between microseconds to milliseconds), bistability and switching threshold. However, there were also some drawbacks, such as, the tendency for mechanical damage to uniform alignment leading to a low electro-optic contrast ratio, charge accumulation in the presence of a net DC field leading to, among other problems, image sticking in displays. Because AFLCs 14 exhibit two field-induced ferroelectric states with apparent tilt angle ± θ in response to application of positive and negative field, the two bistable states can be used alternately with the polarizer oriented at effective θ = 0. With the analyzer oriented at 90, the two ferroelectric states give the same electro-optic response, providing an elegant solution for the DC balance problem. AFLCs also show many of the advantages of SSFLC devices, including fast in-plain optic axis switching, a wide viewing angle, excellent gray scale capability, inherent DC compensation (as mentioned above), driving voltage acceptable for integrated drivers, and the ability of being addressed at video rate by passive matrix schemes. 10

24 However, AFLCs have never been commercialized. One key problem is the low contrast ratio obtainable due to leakage of light in the dark state. This is attributed to the development of a horizontal chevron layer undulation obtained upon driving the sample, and resulting from layer tilt that occurs as the smectic layers shrink on transition from the untilted SmA* to the tilted SmC A * phase. An elegant solution to the problem layer undulations can be found in AFLCs with a tilt angle near 45, which are called orthoconic antiferroelectric LCs (OAFLCs). In such materials the optic axis in the AFLC state is normal to the substrates, effectively making layer undulations invisible, and leading to a clean dark state in the zero field state. Since formation of layer undulations and the non-uniform alignment of AFLCs are still unsolved problems, and had virtually stopped industrial device development of AFLCs, the discovery of orthoconic AFLCs gives hope for future progress in AFLC applications development. Although several OAFLC materials and complex multicomponent mixtures have been reported, 15,16,17 the development of new classes of mesogens showing the OAFLC phase, and developing an understanding LC structure-property relationship in the OAFLCs, are attractive goals, and will be discussed in Chapter Bent-core Liquid Crystals Bent-core Liquid Crystals Bent-core LCs, also known as bow-shaped or banana-shaped LCs (Figure 1.1 and Figure 1.5), have a chevron-shaped aromatic core. In 1996, Niori et al. 18 experimentally demonstrated that bent-shaped molecules show polar order 19 and chiral superstructures 20 in their LC mesophases, despite being formed from achiral molecules. This discovery immediately provoked great interest in the LC research community. 11

25 As shown in Figure 1.5, many bent-core mesogens have C 2v point group symmetry, and such molecular structures are believed to lead to supramolecular structures with macroscopic spontaneous polarization in the smectic layers along a macroscopic C 2 axis. The molecular director n in such bent-core mesogens is considered to be along the long axis of the mesogens. In the case of the typical 120 bend angle, the director n is along the bow string of a bow shaped structure, where the C 2 axis is along the arrow b (Figure 1.5). Even bent-core mesogens lacking molecular C 2v symmetry self-assemble in such a way that the local symmetry of a smectic layer can also be regarded as C 2v, because there is no macroscopic polar orientation of the director, and no polar order along the layer normal, as is true for almost all LC phases. The bent shape effectively results in a polar packing of molecules, which is thought to arise simply due to optimal packing of the molecules within the layers, with net orientation of the b. Figure 1.5 The chemical structure of the classic bent-core mesogen series P-n-O-PIMB (left) and symbolic representations of the bent-core molecules with the shape of an arrow (center). The vertical white arrow in the center figure represents the molecular director (n), and the black arrow represents the two-fold symmetry axis (the arrow of the bow and arrow) (b). 12

26 Due to the bent shape of the cores, these mesogens do not show average C symmetry along the molecular long axes as mesogens in the nematic and SmA phases do. Indeed, bent-core mesogens are capable of producing unique smectic phases that calamitic LCs can not form. During the early stages of the investigation of the phases formed by bent-shaped mesogens, the phases were classified based upon polarized optical microscopy, X-ray diffraction, and dielectric spectroscopy, and designated B 1, B 2 B 7, in order of the chronology of their first characterization. All these phases are often termed banana phases. The B 2, B 4 and B 7 phases are the focus of the present work B 2 Phase The B 2 phase is the most extensively studied banana phase. Niori et al. first observed ferroelectric-like switching current due to the spontaneous polarization along b in the B 2 phase, and attributed such behavior to the close packing of molecules in smectic layers with a unique bending direction along the C 2v symmetry axis. 19 Later, Link and coworkers studied the B2 phase in more detail, and proposed a model structure with C 2 macroscopic symmetry now generally accepted as correct. 19 The B 2 phase actually exists as a family of four sub-phases, SmCP phases, consisting of two conglomerates (SmC A P A and SmC S P F phases) and two racemates (SmC A P F and SmC S P A phases), as indicated in Figure 1.6. The subscripts S and A refer to synclinic or anticlinic tilts of the director in adjacent layers, respectively. The subscripts F and A refer to a ferroelectric or antiferroelectric relationship between adjacent layers, respectively. 13

27 Figure 1.6 In the SmCP phase, three planes, the tilt plane (xz), the layer plane (xy) and polar plane (yz) are defined as shown in (a). Four diastereomeric members of the B 2 family of sub-phases (SmCP phase) are showed in (b). In the SmC A P A and SmC S P F phases, the layer chirality (indicated by blue and red) is identical in adjacent layers. In the SmC A P F and SmC S P A phases, the layer chirality alternates from layer to layer, providing net achiral macroscopic racemates. The original B2 phases described in the literature were antiferroelectric. To obtain a ferroelectric SmCP phase, Walba and coworkers successfully drove the molecular clinicity projected on the polar plane to syn-clinic (see Figure 1.6 (a)). 21 It was pointed out that the tails of molecules in adjacent layers would be anticlinic in the project onto the polar plane in the two 14

28 ferroelectric SmCP phases (see Figure 1.6 (b)). Similar to the situation in calamitic LC systems, materials exhibiting the SmC A * phase are rare compared to those showing the SmC* phase. It was hypothesized that introduction of the same functionality in the tail that leads to antiferroelectric SmC A * materials, most importantly the 1-methylheptyloxycarbonyl tail (MHOC), would lead to ferroelectric B2 phases. Unichiral or racemic MHOC tails often produce anticlinic methyl-terminated layer interfaces in the tilt plane when incorporated into the calamitic LC structures. 22 And indeed, introduction of a racemic 1-methylhepyloxycarbonyl tail into the basic P-9-O-PIMB molecular structure resulted in a ferroelectric phase, later determined to be a variant of the B2 structure possessing spontaneous polarization splay modulation (now termed the B7 phase). The rational design of materials exhibiting the anticlinic SmC A * phase and the current state of understanding how molecular structures determine the observed LC phases will be discussed in Chapter Discotic Liquid Crystals In 1977, Chandrasekhar et al. first reported that disc-like (discotic) molecules exhibit thermotropic mesomorphism, forming columnar phases. 23 Later, Dubois, Levelut, and coworkers further studied LCs formed by discotic mesogens. 24,25 Today, with LC displays having become an essential part of everyday life, it seems clear that discotic columnar LCs cannot compete with calamitic LCs in terms of electro-optic performance. However, due to their unique structural and electronic properties, the discotic LCs show a different constellation of possible applications, including applications in molecular electronics and high-efficiency organic photovoltaics. 26,27,28 Discotic liquid crystals (DLCs) have thus received continuously increasing attention since their initial discovery. 15

29 Simple discotic mesogens typically have a more or less rigid planar core with six or eight (or sometimes four) flexible chain substituents laterally attached to the core. Discotic molecules typically self-assemble into columnar or nematic LC phases. The structures of the columnar LC phases can be described as packing of molecular discs into cylinders. The cylinders represent the average shape of the molecules in the liquid-crystalline packing since the mesogens in the fluid phases are basically free to rotate around their molecular axes. Similar to the nematic structure in calamitic mesogens, for discotic nematic mesophases (N D ), the flat molecules possess long-range orientational order with full positional and rotational freedom around their short axis (normal to the plane of the disk) along n, whereas their two orthogonal long axes (spanning the plan of the discotic mesogen) orient, on average, parallel to a general plane (Figure 1.7). 29 In some cases the discotic mesogens pile up into extended onedimensional (1D) columns, which tend to align along their column axes n, parallel to each other, but with no intercolumnar positional order, forming the so-called columnar nematic phase (N C ). Thus the columns are supramolecular rods similar to the calamitic molecules in a conventional nematic phase. Interestingly, when a disk-shaped electron donor is doped with an electron acceptor, ordered columns can still form as a result of the charge-transfer interaction. For example, alkynylbenzenes doped with 2,4,7-trimitrofluorenone (TNF) forms a columnar naematic N C phase

30 Figure 1.7 Different types of nematic phases formed by discotic mesogens: a) Illustration of a conventional nematic phase of calamitic mesogens; b) Discotic nematic phase N D ; and c) Columnar nematic mesophases (N C ) induced by the charge-transfer interactions between a disk-shaped donor (black) and an electron acceptor (grey). The respective building blocks of the phase (disk-shaped molecules or columns) show long-range orientational order with no long-range positional order, so that all these nematic phases have the same symmetry. 29 The self-organization of discotic molecules into 1D columns is characteristic of the columnar LC phases. Depending on the details of the intracolumnar interactions, there are disordered columns with an irregular stacking of the disks, and ordered columns in which the cores stack in a regularly ordered (equidistant) fashion. Also, the discotic mesogens can be tilted with respect to the column axis n. In the columnar (COL) phases, the columns are arranged in a 2D lattice with the columns parallel to each other. The types of nematic and COL phases formed by discotic mesogens are characterized by their intracolumnar long-range positional order (disordered, ordered and tilted), and the symmetry of the 2D intercolumnar lattice (hexagonal, rectangular, or oblique, as illustrated in Figure 1.8)

31 Figure 1.8 Plane views of the 2D lattices in hexagonal (a), rectangular (b-d), and oblique (e) columnar mesophases. Point-group symmetries in parentheses are according to the international system. 29 In this thesis, work related to columnar LC phases is described in Chapter Experimental Techniques Several techniques are commonly used to study LCs. In addition to the typical techniques used to characterize organic molecules, including proton and carbon nuclear magnetic resonance spectrometry ( 1 H-NMR and 13 C-NMR), mass spectrometry, and elemental analysis, there are techniques which allow characterization of the supramolecular structure and properties of LCs rather than individual molecular structure/properties. An unbiquitous method for characterization of LCs involves observation of the textures obtained by polarized optical microscopy (POM). This technique relies on the optical 18

32 anisotropy and birefringence shown by most LC phases. Experimentally, POM is accomplished as follows. As indicated by Figure 1.9, when unpolarized light passes through a linear polarizer, linearly polarized light will be produced. When the polarized light passes through an optically anisotropic medium, it will be split into ordinary and extraordinary rays. Since the two rays travels at different velocities, they will be out of phase when they recombine. As a result, the output ray will become elliptically polarized. If two polarizing plates are placed with their fast axes oriented at 90, an isotropic material between them appears black (extinguishing state) under POM, since light with one polarization passes through the first plate and the sample unchanged, and is therefore extinguished by the second plate. If a birefringent sample (i.e. LCs) is placed between crossed polarizers with the fast axis of the sample neither parallel nor perpendicular to the first polarizer, the polarization of the light will rotate, allowing light to pass through the second polarizer (usually called the analyzer). For a uniaxial sample, the birefringence ( n) is defined as the maximum difference between the two refractive indices exhibited by the material. The optical properties of a material can be visualized using the index ellipsoid, with three principal axes representing the principal refractive indices of the medium. If the system is isotropic, the index ellipsoid becomes a sphere. Most common LC phases are optically uniaxial. That is, the symmetry of the supramolecular structure of the material is such that it has an axis of symmetry with all perpendicular directions optically equivalent. The axis is defined as the optic axis of the material with two principal refractive indices n o (the refractive index for the light propagating along the optic axis) and n e (the refractive index for the light propagating perpendicular to the optic axis). The magnitude of the optical anisotropy, also called birefringence, is n = n o n e for a material with positive birefringence (birefringence n = n 1 n 2 indicated in Figure ). 19

33 Figure 1.9 The presence of an anisotropic transparent birefringent sample between two crossed polarizing plates will allow light entering through the first plate to exit the second plate. 31 For texture observation by POM, liquid crystal materials are usually sandwiched between two isotropic glass slides. A typical LC cell is composed of two glass slides separated by a distance called the cell gap of the cell (gap between the top and bottom glass slides, and thickness of an LC sample filled into the cell gap). The cell gap is often maintained using spacers, which maintain the gap at a thickness on the order of microns. The glass slides are often glued together at two edges using an epoxy glue. LC materials are filled into the cell by capillary action in the isotropic state. In order to measure electro-optic behavior, the two glass slides are coated in the inner surfaces with transparent electrodes, usually a thin layer of indium tin oxide (ITO). Although molecules have long range orientational order in mesophases, the orientation is only uniform in local domains on the order of 1 µm 2 in size. Multi-domain textures are usually observed in bulk samples in LC cells on the order of 1 cm 2 area. Surface alignment techniques are widely used to obtain uniform planar or homeotropic alignment for the calamitic nematics and smectics. Many simple LC materials have an aromatic core and alkyl or alkoxy chain tails. Such molecules tend to lie down on hydrophilic surfaces and tend to stand up on hydrophobic 20

34 surfaces. By using hydrophobic surfaces such as hydrophobic self-assembled monolayers (SAM) of organic alkylsiloxanes on glass, or spin-cast or dip-coated polymers such as specially functionalized polyamic acids heated to form polyimide (PI) thin films on the substrates, uniform homeotropic alignment, in which the director n is normal to the substrate in nematic and SmA phases, or the layer normal is more or less normal to the substrate in the tilted SmC phase, can be obtained. The most widely used technique for achieving a uniform planar alignment is to mechanically rub LC cell substrates coated with thin, hydrophilic polymer layer. Several polymers are used for the alignment layers, including conventional polyimide, nylon or Teflon. In addition, the director of rod-like LC molecules in nematic or smectic phases can be aligned by application of electric fields as well, due to their well-known dielectric anisotropy. As for the bent-core and columnar LCs, methods for uniform alignment are not as well-established. Differential scanning calorimetry (DSC) is a thermoanalytical technique for detecting phase transitions with high accuracy. Both the sample and reference are sealed in a small aluminum pan, placed in a chamber and heated or cooled at a controlled rate. When the sample undergoes a phase transition, more or less heat will need to flow to it than the reference to maintain both at the same temperature. The difference in the amount of heat required to increase the temperature of the sample and reference is plotted as a function of temperature or time to produce DSC curves. The curve can be used to calculate enthalpies (and therefore entropies) of the transitions by integrating the peak corresponding to a given transition. In the study of LCs, generally there are two types of phase transitions with characteristic DSC curves. The first-order phase transition shows a relatively large peak, corresponding to a large enthalpy change. The second-order phase transition shows a relatively small peak, due to a very small, though almost always observable, enthalpy change (Figure 1.10). 21

35 Figure 1.10 An example of a DSC curve, showing two peaks caused by first-order phase transitions. Powder X-ray diffraction (XRD) is a useful and powerful method used to investigate the mesoscopic structure of soft materials. The orientation and positional ordering in LC phases can be observed by XRD, and it is is possible to deduce structural information speaking to the degree of positional correlation that exists along specific directions in LC phases. For example, for the smectic phases, an electron density wave along the layer normal shows high enough contrast to produce a sharp diffraction at the special angle determined by Bragg s law, 2d sin = n, where d is the layer spacing, is the incident angle, n is integer number, and is the wavelength of the X-rays. In this thesis, for smectic phases, the key characteristic determined by XRD is the smectic layer spacing; for columnar LC phases, the dimensions of the 2D lattice, and the detailed structures of the mesophases are determined by XRD. 22

36 References [1] P. J. Collings, Liquid Crystals, Nature s Delicate Phase of Matter, Princeton University Press, [2] R. B. Meyer, L. Liebert, L. Strzelecki, P. Keller, J. Phys. (France), 1975, 36, L-69. [3] G. Scherowsky, X. H. Chen, Liq. Cryst. 1994, 17, 803. [4] H. Bock, W. Helfrich, Liq. Cryst. 1995, 18, 707. [5] J. Malthete, A. Collet, J. Am. Chem. Soc. 1987, 109, [6] E. Gorecka, D. Pociecha, J. Mieczkowski, J. Matraszek, D. Guillon, B. Donnio, J. Am. Chem. Soc. 2004, 126, [7] K. Kishikawa, S. Nakahara, Y. Nishikawa, S. Kohmoto, M. Yamamoto, J. Am. Chem. Soc. 2005, 127, [8] D. Miyajima, F. Araoka, H. Takezoe, J. Kim, K. Kato, M. Takata, T. Aida, Science 2012, 336, 209. [9] N. A. Clark, S. T. Lagerwall, Appl. Phys. Lett. 1980, 36, 899. [10] J. Valssek, Phys. Rev. 1920, 12, 537. [11] A. D. L. Chandani, T. Hagiwara, Y. Suzuki, Y. Ouchi, H. Takezoe, A. Fukuda, Jan. J. Appl. Phys. 1988, 27, L729. [12] A. D. L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe, A. Fukuda, Jan. J. Appl. Phys. 1989, 28, L1265. [13] D. Engström, P. Rudquist, J. Bengtsson, K. D havé, S. Galt, Opt. Lett. 2006, 31, [14] H. Takezoe, E. Gorecka, M. Čepič, Rev. Mod. Phys. 2010, 82, 897. [15] R. Dąbrowski, J. Gąsowska, J. Otón, W. Piecek, J. Przedmojski, M. Tykarska, Displays 2004, 25, 9. [16] A. Spadło, N. Bennis, R. Dąbrowski, X. Quintana, J. M. Otón, M. a. Geday, Opto- Electron. Rev. 2007, 15, 60. [17] M. Żurowska, R. Dąbrowski, J. Dziaduszek, K. Garbat, M. Filipowicz, M. Tykarska, W. Rejmer, K. Czupryński, A. Spadło, N. Bennis, J. M. Otón, J. Mater. Chem. 2011, 21, [18] T. Niori, T. Sekine, J. Watanabe, T. Furukawa, H. Takezoe, J. Mater. Chem. 1996, 6, [19] D. R. Link, D. M. Walba, Science 1997, 278,

37 [20] T. Sekine, T. Niori, J. Watanabe, T. Furukawa, S. W. Choi, H. Takezoe, J. Mater. Chem. 1997, 7, [21] D. M. Walba, E. Korblova, R. Shao, J. E. Maclennan, D. R. Link, M. A. Glaser, N. A. Clark, Science 2000, 288, [22] A. D. L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe, A. Fukuda, Jpn. J. Appl. Phys. 1989, 28, L1265. [23] S. Chandrasekhar, B. K. Sadashiva, K. A. Suresh, Pramana, 1977, 9, 471. [24] J. Billard, J. C. Dubois, N. Huutinh, A. Zann, Nouv. J. Chim. 1978, 2, 535. [25] A. M. Levelut, J. Phys. Lett. 1979, 40, L81. [26] L. Schmidt-Mende, a Fechtenkötter, K. Müllen, E. Moons, R. H. Friend, J. D. MacKenzie, Science 2001, 293, [27] J. Nelson, Science 2001, 293, [28] S. Xiao, M. Myers, Q. Miao, S. Sanaur, K. Pang, M. L. Steigerwald, C. Nuckolls, Angew. Chem. Int. Ed. 2005, 44, [29] S. Laschat, A. Baro, N. Steinke, F. Giesselmann, C. Hägele, G. Scalia, R. Judele, E. Kapatsina, S. Sauer, A. Schreivogel, M. Tosoni, Angew. Chem. Int. Ed. 2007, 46, [30] K. Praefcke, D. Singer, B. Kohne, M. Ebert, A. Liebmann, J. H. Wendorff, Liq. Cryst. 1991, 10, 147. [31] 24

38 2 Odd-Even Behavior of Ferroelectricity and Antiferroelectricity in Tricarbosilane-Terminated Liquid Crystals 2.1 Introduction Ferroelectric and antiferroelectric smectic LCs are considered promising candidates as alternatives to nematic LCs for flat-panel displays. AFLCs attracted much attention because they seemed to solve some issues that the SmC* SSFLCs have, including limited grey scale capability and dc-compensation. However, AFLCs are found to be even more difficult to align than FLCs, which results in light leakage in the dark state and poor contrast. In this regard, orthoconic antiferroelectric FLCs 1,2 (OAFLCs = AFLCs with θ = ±45 ) show some unique and attractive features, most importantly a near ideal dark state even with nonuniform orientation of the smectic layers. Specifically, in order to obtain this orthoconic, near dark state, the helix of the SmC A * must be suppressed, e.g. by means of surface-stabilization. In order to obtain the surfacestabilized state, the gap of cells must be smaller than the helical pitch of OAFLCs. So for further OAFLC development, the most important goals and requirements of materials design and synthesis are: optical tilt ±45 ; helix pitch long enough for surface-stabilization; and broad temperature range of the orthoconic phase, including room temperature. The first OAFLC material (W107), comprised of a four-component mixture, was reported by Roman Dabrowski and the Warsaw liquid crystal group 3 (Figure 2.1). W107 exhibits a SmC A * phase between 30 C and 120 C, for which the tilt angle ~±45 for temperatures equal to or lower than 80 C. Later, other OAFLC mixtures were formulated by the same group, showing the OAFLC state at room temperature. For example, W193B is composed of 11 components with 25

39 one partial fluorinated terminal chain. 4,5 To our knowledge all of the Warsaw OAFLC mesogens possess the MHOC tail present in the original antiferroelectric LC MHPOBC (vide infra). Due to the presence of multiple components, it is difficult to control the exact ratios, so that in reference 4, the single layer tilt angle of W193B is 45, whereas in reference 5, it is reported to be 40. The relatively few available orthoconic materials generally exhibit sub-µm values of the helix pitch. Therefore, the orthoconic state can be obtained only in very thin cells, for example the cell gaps have to be about smaller than ~ 1 µm. Such thin cells are difficult to manufacture in a high volume, large area production setting. Moreover, in such thin cells, the LC layer would be too thin to constitute the desired /2-wave plate ( nd = /2) in the field-induced synclinic state. Finally, in such thin cells, the SmC* state becomes metastable, requiring sophisticated drive schemes and slowing the overall response times. 6 Recently, W242 and W252, containing esters with fluorinated moieties in the terminal chain, reported by Warsaw group, exhibit highly tilted AFLC phases with pitch values of >1 µm. 7 Also, for use in active matrix devices the spontaneous polarization (P S ) should be small. These OAFLC materials generally possess P S values of 200 nc/cm 2, a rather large value of P S. 26

40 Figure 2.1 Molecular structures and composition of W107, the first reported OAFLC mixture. 2.2 Goals and Prior Work Understanding liquid crystal (LC) mesogen structure-property relationships, and in particular, how the molecular structures of mesogens determine the observed LC phases, is of fundamental importance, yet represents a challenge, since very small variations in free energy per molecule can have a dominating effect on the collective free energy of the supramolecular self-assembly. In this regard, nanophase segregation is currently under active investigation as an important contributor to LC phase energetics and structures. 8 For example, the Boulder LC group reported that doping low concentrations of bent-core molecules into calamitic smectic materials can induce anticlinic and biaxial smectic phases due to nanophase segregation of bent-core molecules in a smectic solvent. 9 Indeed, simple calamitic lamellar LCs such as the untilted SmA and uniformly tilted (synclinic) SmC (Figure 2.2a), and their chiral counterparts composed of enantiomerically enriched chiral molecules, SmA* and SmC*, can be considered to result from nanophase segregation of aromatic cores and aliphatic tails, forming the well-known lamellar phases with intra-layer nanophase segregation. In the organization process of discotic molecules to form columnar LC superstructures, nanophase segregation takes place between the highly ordered, rigid aromatic cores and the disordered aliphatic chains (Figure 2.3)

41 Figure 2.2 a) Illustration of the local layer structure of the SmC*; and b) The SmC A * phases of the prototype antiferroelectric MHPOBC (molecular structure given in c). The layer interfaces (horizontal lines) in the uniformly tilted SmC structure are denoted as synclinic, while the layer interfaces in the alternately tilted SmC A are denoted as anticlinic. The chiral methylheptyloxycarbonyl grouping is thought to be the key chemical structure feature leading to the rare anticlinic phases such as the SmC A. Figure 2.3 Schematic illustration of the organization process taking place during solidification of alkylsubstituted discotics from the isotropic phase. 10 Inter-layer nanophase segregation has more recently served as an important foundation for hypotheses explaining the occurrence of relatively rare smectics, including the 28

42 technologically interesting and valuable SmC A * as described above, 11,12 with alternating layer tilt (anticlinic, Figure 2.2b) and the de Vries 13 (SmA-SmC transition with anomalously small layer shrinkage) variants of the SmA* and SmC* phases. The observation that the layering presents as being more well-defined in these rare phases is consistent with inter-layer nanophase segregation. For example, in small angle X-ray diffraction (XRD), only a first-order reflection resulting from Bragg scattering from the layers (2d sin = n, n = 1, d is the layer spacing, is the incident angle, and is the wavelength of the X-rays) is seen in most cases. In the SmC A * and de Vries phases, however, harmonics of the fundamental layer peak are usually seen (Bragg law, n>1). 14 A current hypothesis for the effect of nanophase segregation on phase behavior focuses on the entropy of out-of-layer fluctuations in smectic LCs. Thus, in the SmC A * materials also possessing a SmC* phase, the SmC* always occurs at higher temperature, proving it has the higher entropy of formation. Figure 2.2a illustrates the local layer structure of the SmC*; and Figure 2.2b exhibits the SmC A * phases 15 of the prototype antiferroelectric MHPOBC (molecular structure given in Figure 2.2c). The layer interfaces (horizontal lines) in the uniformly tilted SmC structure are denoted as synclinic, while the layer interfaces in the alternately tilted SmC A are denoted as anticlinic. The chiral methylheptyloxycarbonyl group is thought to be the key chemical structural feature leading to the rare anticlinic phases such as the SmC A *. Following Glaser et al., 16 our current working hypothesis for this effect is that the methylheptyloxycarbonyl tail (top tail in Figure 2.2C) and simple variants provide suppression of out-of-layer fluctuations, allowing normally entropically disfavored anticlinic layer interfaces to form. More specifically, the geometry of the MHOC groups tends to orient the alkyl chain 29

43 more or less parallel to the layer plane 17, suppressing out-of-layer fluctuations at the molecular length scale. Also, there is a clear similarity of structural features leading to antiferroelectrics. In a sense, the MHOC effect can be considered a kind of nanophase segregation phenomenon, which isolates the layers and removes much of the proposed entropic benefit of synclinicity. Molecules involving a kind of polyphilicity 8a,18 lead to nanophase segregation. Such molecules have been widely used to provide more complex self-assembled structures 19 and tailor properties of materials by controlling the molecular topology. 20 Polyphilic molecules are generally comprised of three or more chemically different subunits which tend to segregate into homogeneous microdomains. Thus, due to an interlamellar polyphilic effect, molecules with alkyl tails terminated by oligosiloxanes, 21, 22 oligocarbosilanes, 23, 24 or tails composed of fluorinated alkyl groups 25 are known to provide SmC A * phases in calamitics, and SmC S P F and SmAP F phases in bent-core LCs. In a seemingly somewhat related phenomenon, it is reported that interlayer structure leading to ferroelectric and antiferroelectric phases in bent-core mesogens can be controlled by the alkyl spacer lengths between a simple branched hydrocarbon tail-terminating unit and the aromatic core. 26 This is not considered to be a manifestation of nanophase segregation by the authors, but rather a molecular-level steric interaction at the layer interfaces. Several years ago, R. A. Reddy in our group synthesized a mesogen, W586, 24 with properties suggesting that tricarbosilane units are segregated into sublayers, allowing anticlinic ordering at the layer interfaces in an untilted smectic, and representing the first reported orthogonal ferroelectric (SmAP F ) phase (Figure 2.4). The design strategy leading to the SmAP F material involved two parts: 1) The bent-core molecular design employs only a single flexible tail that stabilizes layers with untilted molecules because one tail provides more area per 30

44 molecule for the tail sublayer, and thus lowering their in-plane entropic pressure, eliminating the strong tendency for tilted phases from normal bent core mesogens with two tails; and 2) Terminating the tail with a carbosilane group suppresses the interlayer fluctuation to reduce the tendency for synclinic, antiferroelectric layer ordering. Figure 2.4 (a) Molecular structure of W586 and its phase sequence on heating; (b) The layer spacing obtained from X-ray is about 61 Å, much larger than the calculated extended molecular length of 52 Å, implying a structure in which adjacent molecules have antiparallel packing. The silane termination suppresses out-of-layer fluctuations to promote ferroelectric coupling (anticlinic ordering) of adjacent 31

45 layer; (c) Reducing the number of tails from two to one per molecule creates more space for the tails, promoting orthogonal phases. Out-of-layer fluctuations (ellipse) enable penetration of tails into the adjacent layers favoring synclinic tail tilt at the layer interfaces and thus macroscopic antiferroelectric (SmAP A ) order shown by bent-core structures without a carbosilane or similar group (comparison). Introducing carbosilane into the tail suppresses out-of-layer fluctuations, favoring anticlinic tail orientation and thus the SmAP F structure observed from W Design and Synthesis Herein, we describe the design and synthesis of a new class of nanophase segregated LC homologs 1(n), where n is the number of methylene groups between the oxygen atom connected to the core, and the tricarbosilane units, possessing a mesogenic tolanphenyl carboxylate core, a chiral alkyl chain, and a tricarbosilane-terminated alkyl tail. These homologs exhibit a dramatic odd-even effect, alternating between the chiral ferroelectric SmC* phase with close to 45 tilt, and the rare and potentially useful chiral orthoconic antiferroelectric SmC A * phase. These results are surprising, since isolation of the layers from each other would seemingly make the number of methylenes in the spacer effectively irrelevant as a determinant of layer interface clinicity. Observation of such an effect with perfect fidelity in a nanophase segregated system has not been reported previously, and serves as an interesting data point in understanding of the structure and energetics of nanophase segregated supramolecular self-assemblies. Figure 2.5 Molecular structures of the compounds under investigation. 32

46 Synthesis of 1(n) is described in Scheme 2.1. Mitsunobu coupling of 4-iodophenol with (R)-2-octanol gave (S)-1-iodo-4-(octan-2-yloxy) benzene 2. Methyl 4-ethynylbenzoate 4 was obtained via Sonogashira coupling of methyl-4-iodobenzoate with ethynyltrimethylsilane, followed by deportation of TMS smoothly by TBAF at low temperature in 87% yield. Diphenylethylene 5 was prepared via Sonogshira coupling of ethynylbenzoate 4 with iodo functionality in 2 and then saponification of 5 afforded the carboxylic acid 6. The tricarbosilane tail 7 can be synthesized based on nucleophilic substitution. Chloromethyltrimethylsilane can be successfully converted to a Grignard reagent, which attacked the chloro(chloromethyl)dimethylsilane under a substitution pathway to form chloro dicarbosilane intermediate with excellent yield. A similar reaction that converted chloro dicarbosilane to the corresponding Grignard reagent, and followed by coupling with chlorodimethylsilane would generate the desired tricarbosilane tail 7. Hydrosilylation were carried out between the tricarbosilane 7 and the alkene 8, which can be also obtained under Mitsunobu coupling, to achieve compound 9. Compound 9 was deprotected to afford the corresponding phenol 10 by hydrogenation, which then underwent DCC coupling with the carboxylic acid 6 to accomplish the molecule 1(n) (The value in parentheses indicates the number of methylene segments between the core and the tricarbosilane unit). 33

47 Scheme 2.1 Synthesis of a series of tricarbosilane-terminated compounds 1(n), W617_1(11), W651_1(10), W650_1(9), W696_1(8), W677_1(7), W641_1(6), W676_1(5). 34

48 2.4 Results and Discussion Mesomorphic Properties of 1(n) Homologues All these materials were characterized by differential scanning calorimetry (DSC), polarized optical microscopy and electro-optics (POM and EO), powder X-ray diffraction, and depolarized reflected light microscopy on freely suspended thin films (DRLM). 27 The phase sequences upon heating and cooling, transition temperatures, and clearing enthalpies are summarized in Table 1. As indicated, all 1(n) show broad temperature ranges for enantiotropic smectic phases, even below the room temperature. There is, however, no clear trend, or odd-even effect in the clearing points or clearing enthalpies. Mesogens 1 do show striking odd-even effect of remarkable fidelity: All n = odd members of the series exhibit the close to 45 orthoconic AFLC phase, while all n = even members exhibit the more or less 45 SmC* ferroelectric phase. We can only identify one LC phase for each compound. There are some unidentified phases (denoted as X in Table 2.1) at temperatures below the smectic phases (Figure 2.6). They are likely to be crystallization, or partial crystallization and glass formation at low temperatures. 35

49 36

50 Figure 2.6 DSC traces of 1(n) on second heating/cooling at a scan rate of 2 C min -1. n phases, transition temperatures ( C), and enthalpies (kj mol -1 ) in parentheses [a] Layer spacing (Å) [b] X-7.7 SmC A *-54.5 (4.1) Iso X 0.86-SmC A * 53.6-Iso X-2.87 SmC*-54.1 (4.3) Iso X SmC* Iso X-12.2 SmC A *-67.7 (4.7) Iso X 11.5-SmC A * 66.9-Iso X-12.9 SmC*-71.5 (5.3) Iso X 12.4-SmC* 70.8-Iso X-12.6 SmC A *-74.1 (5.7) Iso X 12.3-SmC A * 72.7-Iso X-15.1 SmC*-74.1 (4.2) Iso X 11.7-SmC* 73.4-Iso X-13.8 SmC A *-79.8 (6.3) Iso X 13.5-SmC A * 79.3-Iso Table 2.1 [a] Structure of the new tricarbosilane-terminated 1(n), phase sequence and transition temperatures, and clearing enthalpy, from a combination of DSC, XRD, POM, EO, and DRLM data; [b] layer spacing at 55 C. In LC cells (one surface rubbed) coated with a thin film of nylon, the odd number homologues of the series exhibit typical, unique EO behavior of OAFLCs, due to surfacestabilized planar alignments of mesogens (Figure 2.7). Without an applied electrical field, a dark state observed by POM in LC cells less than 3 µm thick suggests a typical OAFLC phase (closed to 45 of tilt angles). Application of an electric field causes soliton-wave mediated switching 28 to the ferroelectric state with an expected bright birefringence color, which is attributed to the EO 37

51 switching from SmC A * to SmC*. In thicker cells (~ 3.5 µm), some textures of W676_1(5) are visible in the cell, indicative of helix formation. The fact that the surface stabilized orthoconic texture is obtained in cells up to 3 µm thickness suggests that the helix pitch in these materials is considerably larger than most OAFLCs. 3,4,5 Warsaw group has recently reported relatively longpitch (> 1 µm) orthoconic mixtures. For example, W252 possesses five components. 29 High-tilt ( = 44 ) single compound AFLC material was reported by Sigarev et al, 30 however the SmC A * phase cannot be achieved at room temperature (the temperature range of SmC A * is from 115 to 57.5 C). W617_1(11) W650_1(9) W677_1(7) 38

52 W676_1(5) Figure 2.7 (a) For the odd number homologs, a dark state can be observed off the electrode in surfacestabilized planar aligned LC cells (less than 3 µm thick); In a 3.5 µm-thick cell, some subtle textures were visible, indicative of helix formation. (b) The photomicrograph showing the switching behavior from SmC A * to SmC* under the application of an electric field. (c) The ferroelectric state with the expected bright birefringence can be observed. It has been proven to be impossible to achieve an acceptable dark state between crossed polarizers for AFLCs, which otherwise would have an enormous potential for electro-optic applications, particularly in high-resolution full color displays, which have not yet reached manufacturing, due to the severe intrinsic problem of folds in the smectic layers, which drastically limit the achievable contrast. Conventional surface-stabilized antiferroelectrics are optically positive biaxial crystals, with an effective optic axis along the smectic layer normal. The unique optical property of the corresponding orthoconic antiferroelectrics can be formulated 39

53 since the tilt directions in adjacent smectic layers are oriented perpendicular to each other. The material becomes negatively uniaxial with the optic axis lying perpendicular to the smectic layer normal. The electro-optic effect in such a material is based on the fact that the optic axis can be switched between three mutually orthogonal directions, which correspond to zero, negative, or positive values of the applied electric field. These unique electro-optic properties make the class of OAFLCs promising for application in displays. The odd number homologues of 1(n) created by our group with a tricarbosilane terminal group exhibit unique properties of OAFLC mesophase in wide temperature range even below room temperature. More interestingly, the mesogens with the odd number of alkyl segments allow a cell thickness up to 3 µm to exhibit a dark state and show a spontaneous polarization P s of about 50 nc/cm 2. This is pretty much lower than the previously reported OAFLC mixtures ( 200 nc/cm 2 ). The measurement of P s will be discussed later in this chapter. In contrast, all homologues with the even number of methylene units exhibits the synclinic SmC* ferroelectrics with nominal domain-wall mediated, 31 FELC electro-optics in planar aligned cells, as shown in Figure 2.8. Figure 2.8 The photomicrograph showing domain-wall mediated behavior under application of an electric field +0.5 V/µm to a 3.1- m-thick cell of the homologues with the even number of alkyl segments W651_1(10), W696_1(8), W641_1(6). 40

54 In 4 µm-thick commercial homeotropic cells, odd and even homologues of the series also exhibit totally different behavior. All homologues exhibit focal conic textures when cooling from the isotropic phase to the dark state. But, the odd number antiferroelectric homologues show an interesting dynamic texture that is not observed in the even homologous while evolving from the focal conic to the dark texture. It is proposed that this dynamic texture results from the helix pitch in the orthoconic phase changing with temperature. The ferroelectric and antiferroelectric assignments for all of the homologues of 1(n) are fully confirmed by DRLM. 27 The experiment involves application of electric fields to thin (several layers; both odd and even layer number) freely-suspended smectic films, and observation of the response of the samples by reflected light microscopy. In order to analyze the results of DRLM, a short introduction of DRLM will be given. Freely-suspended films are an effective method to produce uniformly oriented smectic layers. To prepare the freely-suspended films, a mesogen is placed on a glass slip with a circular hole of around 4 mm in diameter, and heated to a temperature just below the isotropic-to-smectic transition temperature. Another cover slip is used to draw the material across the hole. Using this technique, one to several thousand layers can be formed. The thin films can exist with only air on top and bottom. Since most LC molecules are aligned perpendicular to the air interface, homeotropic orientation of molecules can be obtained in thin films with large areas. An electric field parallel to the smectic layers is applied. In order to probe coupled in-layer-plane molecular orientation and electric polarization fields on the thin film, the direction of molecular tilt and spontaneous polarization have to be emphasized. On a layer at the air interface, the C 2 axis in the plane of the layer no longer exists. The air LC interface reduces the symmetry, allowing surface polarization along the molecular long axis at the air interface. 27 As shown in Figure 2.9, the 41

55 different number of layers in the anticlinic or clinic state generates polarization in different directions. 42

56 Figure 2.9 Molecular orientations in an odd number and even number of smectic layers in SmC* and SmC A * phases. Green marks indicate transverse polarization (the net spontaneous polarization normal to the tilt plane) and yellow arrows indicate longitudinal polarization (the net polarization parallel to the tilt plane). (a) As for SmC* films, both of odd number and even number of smectic layers have a net transverse polarization. (b) As for SmC A * films, films with an even number of smectic layers have a net longitudinal polarization while films with an odd number of layers have a net transverse polarization. The photomicrograph of DRLM verifies that W617_1(11) is an antiferroelectric material. In Figure 2.10, the regions with an even numbers of layers, such as the region a, are bright, implying that the molecules are longitudinally polarized when the analyzer has been decrossed towards the electric field. Thus the tilt plane must lie parallel to the polarization. These dark regions with odd numbers of layers, such as the region b, imply that the polarization of the molecules must fall in the second or fourth quadrants. Thus the tilt plane must be perpendicular to the polarization to give transverse polarization. All the odd-carbon homologues exhibit polarization normal to the tilt plane (transverse polarization) for odd layer numbers, and polarization in the tilt plane (longitudinal polarization) for even layer numbers, which is characteristic of SmC A * phase. 43

57 Figure 2.10 Regions with odd (o) and even (e) number of layers of freely suspended W617_1(11) film at T = 70 o C, analyzer is decrossed by +3 o and an applied square wave electric field of E = 28.5 V/mm with a frequency of 0.2 Hz. The photomicrographs of DRLM fully confirm that W651_1(10) is a ferroelectric material. In Figure 2.11, there are regions of varying number of layers, both even and odd. Through laser reflectivity measurements, the thicknesses of 3, 4, 6 and 7 layers were determined. Because all of these regions appear brighter in Figure 2.11a when the analyzer is decrossed away from the electric field, and these regions turn darker in Figure 2.11b when the analyzer is decrossed towards the electric field, the molecules in these layers must have transverse polarization (molecular tilt plane perpendicular to an applied electric field). As pictured in figures, regions with an odd and even number of smectic layers all have transverse polarizations, allowing that W651 is identified to be a ferroelectric smectic phase (SmC*). 44

58 Figure 2.11 Regions of varying layer thicknesses at T= 40 o C with an applied square wave electric field of E=0.057 V/mm with a frequency of 0.2Hz. The number of layers of each region is labeled on the images. a) Analyzer is decrossed +20 o (away from the electric field). Regions appear bright since molecules have transverse polarization. b) Analyzer is decrossed -20 o (towards the electric field). High-resolution X-ray diffraction experiments accomplished on the Brookhaven synchrotron light source confirm the presence of a relatively well-defined layer structure by observation of a second harmonic peak of the layer reflection. 32 Harmonics of the layer reflection are typically not seen in conventional smectic LCs. In Figure 2.12, a second harmonic peak resulting from Bragg scattering from the layers of the mesogens of W617_1(11) and W651_1(10) are consistent with interlayer nanophase segregation, which is attributed to the sublayer formation by the tricarbosilane units. 45

59 Figure 2.12 A second harmonic of the layer peak in the W617_1(11) and W651_1(10). The layer thickness, which is in line with highly tilted arrangement of the molecules, is close to 45 relative to the layer normal. The layer thickness is always significantly smaller than molecular length, which is in line with highly tilted arrangement of the molecules. For example, the layer spacing of 1(11) (d 4 nm) is smaller than the fully stretched molecular length of about 5 nm for 1(11). 33 Figure 2.13 shows the layer spacings, d versus temperature during LC phase transition. There seems to be a trend in the layer spacing measured by XRD, with the C11 and C10 homologues, and the C7 and C6 homologues showing very similar layer spacings, with a large jump down in spacing from C10 to C9, and from C6 to C5. However, the C9 and C8 spacings are different, breaking the trend. In addition, the layer spacing could be influenced by the small variation of the tilt angle. Figure 2.13 X-ray diffraction measurements of layer spacings of all mesogens in this series, denoted by d, as function of T. 46

60 Polarization(nC/cm 2 ) Polarization measured as a function of T on cooling for the mesogens in this series is shown in Figure OAFLC materials with the values of polarization, about 50 nc/cm 2, indicate the potential application in active matrix devices W676_1(5) W641_1(6) W677_1(7) W696_1(8) W650_1(9) W651_1(10) W617_1(11) Temperature ( o C) Figure 2.14 Polarization of all mesogens in this series, as a function of T on cooling Nanophase segregated silane sublayer The antiferroelectric and ferroelectric phases appear alternatively when the number of methylene groups between the aromatic core and terminating tricarbosilane group is odd and even, respectively. This interesting odd-even effect prompts us to propose a plausible model to explain these behaviors, as shown in Figure Our hypothesis is based on the assumptions that the tricarbosilane moieties build up their own nanophase segregated sublayers at the interfaces. The current working hypothesis is described as follows. We consider that the tricarbosilane units in the silane sublayer are in an all-anti, elongated shape, and more or less parallel to each other. If the nematic director in the tricarbosilane sublayer is oriented along the 47

61 overall director of the aromatic cores, synclinic layer interfaces are expected (Figure 2.15a). If the nematic director in the tricarbosilane sublayer is normal to the layer interfaces, the anticlinic layer interfaces would be reasonable, though not necessary, as illustrated in Figure 2.15b. Figure 2.15 Proposed of the organization of 1(n) when n is even number (a) and when n is odd number (b) in the triply segregated smectic phase. This model then requires that the orientation of tricarbosilane units in their sublayers must be controlled to some extent by the number of methylene groups in the alkyl spacers. In order to gain some insights on the odd-even behavior, we conducted the conformational analysis in single molecules in the gas phase. HF631G* calculations of reasonable conformational minima of 1(n) are interesting (these do not change significantly with a density functional model (B3LYP 631G*). If the alkyl and carbosilane chains are all anti, then odd carbon numbers put the silane director along the molecular director, while the even carbon numbers have the silane director normal to the layer interfaces. This seems at odds with the experimental results. However, if a gauche bend is introduced at the OC-CC bond on both sides of the core, the 48

62 geometry flips, as can be seen with the conformations shown in Figure We have used this kind of gauche bend to interpret the sign and rough magnitude of the ferroelectric polarization of SmC* materials. 34 And gauche conformers of n-alkyl chain were also found in the liquid crystal state of a macrocyclic phosphate. 35 Figure 2.16 One conformer of 1(11) (a) and 1(10) (b) for illustration of the models in Figure In conclusion, we designed and synthesized a new class of smectic mesogens 1(n) possessing a tolanphenyl carboxylate core, a chiral alkyl chain, and a tricarbosilane-terminated side chain. The phase structures of 1(n) have been studied by POM and electro-optics, DRLM, and X-ray diffraction. We found an odd-even behavior with perfect fidelity for the alternative appearances of ferroelectric and antiferroelectric LC phases. More interestingly, all odd mesogens exhibits the rare and useful orthoconic antiferroelectric LC phase. The hydrocarbon spacer separating the nanophase segregated tricarbosilane sublayers from the aromatic core controls the clinicity of the layer interfaces. Investigations on such odd-even behavior in 49

63 tricarbosilane-terminated bent-core mesogens are currently under way in our group. Our strategy of segregating aliphatic chains and polyphilic moieties, such as tricarbosilane units, to control the supramolecular self-assembly can guide the new molecular designs for self-organizations to exhibit desired properties in bulk. The discovery of the odd-even behavior highlights the importance of the lengths of alkyl chains, which are usually regarded as innocent linkers. These findings also point out that the supramolecular self-assembly of polyphilic molecules may be engineered by lengths of hydrocarbon spacers. After the unexpected discovery of the odd-even effect in calamitic mesogens, it posted the question of whether the control of the calamitic interlayer clinicity could be extended to control the ferroelectricity and antiferroelectricity in bent-core mesogens with tricarbosilaneterminated units? W586 homologues with a tricarbosilane at one end were chosen as the test. As shown in Figure 2.17, W586 possesses eleven methylene spacers between the core and the tricarbosilane unit, showing SmAP F. Maria Kolber proceeded to synthesize W586 homologues from ten to seven methylene spacer. Odd-even effects with perfect fidelity were observed as well: the materials with an odd number of methylene units exhibited SmAP F ; while the materials with an even number showed SmAP A. 50

64 Figure 2.17 The molecular structures and bent-core structures of W586 homologues. 2.5 Experimental Procedures All glassware was oven-dried or flame-dried. CH 2 Cl 2 and toluene were distilled from CaH 2 under nitrogen; THF were distilled from sodium benzophenone ketyl under nitrogen. Unless specifically mentioned, all chemicals are commercially available and were used as received. Flash chromatography was performed using 60 Å silica gel (37-75 μm). All compounds in the synthetic route were routinely characterized by NMR spectroscopy. 1 H and 13 C NMR spectra were recorded using a Varian Unity INOVA-500, a Bruker AM-400 spectrometer, or a Bruker 300 UltraShield. 1 H NMR spectra are reported in parts per million (δ) relative to residual solvent peaks (7.26 for CDCl 3 ). 13 C NMR spectra are reported in parts per million (δ) relative to residual solvent peaks (77.23 for CDCl 3 ). Phase transition temperatures were determined by DSC using a Mettler Toledo DSC823. POM data were collected using a Nikon-HCS400 microscope 51

65 with an Instec STC200 temperature-controlled stage. X-ray diffraction experiments were temperature controlled with an Instec STC200 hotstage, and data were collected using a point detector mounted on a Huber four-circle goniometer at either of the following: Synchrotron radiation at beamline X10A of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (High Resolution); or using Cu K(α) radiation from a Rigaku UltraX-18 rotating anode generator, operated by the Liquid Crystal Materials Research Center, University of Colorado Boulder (Low Resolution). (S)-1-iodo-4-(octan-2-yloxy) benzene 2 To a solution of 4-iodophenol (3.0 g, mmol), triphenylphosphine (4.65 g, mmol) and (R)-2-octanol (3.25 ml, mmol) in THF at room temperature was added DEAD (3.22 ml, mmol). The mixture was stirred overnight, quenched by the addition of sodium hydroxide solution (0.5N, 50 ml) and extracted with ether (50 ml 3). The combined organic layers were washed with brine and dried over Na 2 SO 4 and concentrated under reduced pressure. The crude product was purified by column chromatography (EtOAc/Hexane 1:49) to give (S)-1-iodo-4- (octan-2-yloxy) benzene 2 (4.3 g, 94%). 1 H NMR (400 MHz, Chloroform-d) δ (m, 2H), (m, 2H), 4.32 (h, J = 6.1 Hz, 1H), (m, 1H), (m, 1H), (m, 11H), 0.92 (t, J = 6.7 Hz, 3H). 52

66 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , 82.42, 74.14, 74.12, 36.50, 31.96, 29.43, 25.63, 22.78, 19.76, (more peaks are observed due to H-C coupling). Methyl 4-((trimethylsilyl)ethynyl)benzoate 3 Methyl 4-iodobenzoate (3.0 g, mmol), CuI (221.8 mg, 1.16 mmol), and Pd(PPh 3 ) 2 Cl 2 (401.8 mg, 0.57 mmol) were charged in a 100 ml sealed tube, which was purged with Ar. Triethylamine (6.0 ml) in THF (30 ml) solution was degassed by sparging with Ar for 15 min and cannulated into the tube. Ethynyltrimethylsilane (1.94 ml, mmol) was then added via syringe. The mixture was heated at 80 o C for 2h and then cooled to room temperature, and filtered. The filtrate was concentrated under reduced pressure. The crude product was purified by column chromatography (CH 2 Cl 2 /Hexane 1:2) to give methyl 4-((trimethylsilyl)ethynyl)benzoate 3 (2.6 g, 98%). 1 H NMR (400 MHz, Chloroform-d) δ (m, 2H), (m, 2H), 3.88 (s, 3H), 0.25 (s, 9H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , 97.50, 52.06, Methyl 4-ethynylbenzoate 4 53

67 A 1.0 M solution of tetrabutylammonium fluoride (12 ml) was added to a stirred solution of methyl 4-((trimethylsilyl)ethynyl)benzoate 3 (2.7 g, 11.6 mmol) in THF at -78 o C. After stirring for 20 min, the reaction mixture was stirred at 0 o C for 1h. The reaction mixture was diluted with EtOAc, and then quenched by addition of sat. NH 4 Cl (50 ml). The biphasic mixture was extracted with EtOAc(50 ml 3), and the combined organic layers were washed with brine, dried over Na 2 SO 4, and concentrated under reduced pressure. The crude product was purified by flash chromatography (1:9 EtOAc:hexane) to provide 4 (1.6 g, 87%). 1 H NMR (400 MHz, Chloroform-d) δ (m, 2H), (m, 3H), 3.85 (s, 4H), 3.23 (s, 1H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , 82.83, 80.29, (S)-methyl 4-((4-(octan-2-yloxy)phenyl)ethynyl)benzoate 5 Methyl 4-ethynylbenzoate 4 (1.56 g, 9.73 mmol), Pd(PPh 3 ) 2 Cl 2 (341.6 mg, 0.49 mmol), and CuI (185.3 mg) were charged in 100 ml sealed tube and purged with Ar. The solution of (S)-1-iodo- 4-(octan-2-yloxy) benzene 2 (3.64 g, mmol) and Triethylamine (18.4 ml) in 40 ml THF was degassed by sparging with Ar for 15 min and then cannulated into the sealed tube. The mixture was heated for 20 h at 45 o C, and then cooled to room temperature. The heterogeneous 54

68 mixture was filtered through a short pad of celite and concentrated under reduced pressure. Purification by flash chromatography (5:95 EtOAc:Hexane) provided 5 (3.1 g, 87%). 1 H NMR (400 MHz, Chloroform-d) δ (m, 2H), (m, 2H), (m, 2H), (m, 2H), 4.35 (h, J = 6.1 Hz, 1H), 3.89 (s, 3H), 1.72 (dddd, J = 11.5, 9.8, 6.3, 3.1 Hz, 1H), (m, 1H), (m, 11H), (t, 3H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , 92.92, 87.47, 73.94, 52.18, 36.46, 31.88, 29.36, 25.55, 22.70, 19.69, HRMS m/z calcd for C 48 H 56 O 6 Li + : ; found: (S)-4-((4-(octan-2-yloxy)phenyl)ethynyl)benzoic acid 6 (S)-methyl 4-((4-(octan-2-yloxy)phenyl)ethynyl)benzoate 5 (3.04 g, 8.51 mmol) was dissolved in THF (30 ml). The LiOH solution (30 ml of aqueous solution, 7.14 g, mmol) was added into the THF solution. The resulted mixture was heated to reflux overnight, cooled to room temperature, and then quenched by addition of 1M HCl. The mixture was extracted with EtOAc (50 ml 3). The combined organic layers were dried over Na 2 SO 4, decanted, and concentrated under reduced pressure to provide the crude acid 6 (2.85 g, 98%). 1 H NMR (400 MHz, Chloroform-d) δ (m, 2H), (m, 2H), (m, 2H), (m, 2H), 4.39 (h, J = 6.1 Hz, 1H), 1.75 (dddd, J = 13.5, 9.9, 6.4, 5.0 Hz, 1H), 1.58 (ddt, J = 13.6, 11.1, 5.3 Hz, 1H), (m, 11H), (m, 2H). 55

69 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , 93.57, 87.55, 74.20, 36.59, 32.00, 29.47, 25.69, 22.82, 19.88, HRMS m/z calcd for C 23 H 25 O 3 - : ; found: Trisilane tail 7 Mg (1.35 g, mmol) was charged in 250 ml three necked round bottom flask, 20 ml THF was added to the flask to barely cover the Mg (1.35 g, mmol), and then the solution of chloromethyltrimethyl silane (5.66 g, mmol) in 15 ml dry THF was cannulated into the flask slowly. The mixture was treated with a small amount of iodine, and heated with heat gun to initiate the reaction. The additional 20 ml THF was cannulated into the reaction mixture slowly. The mixture was heated to reflux for 2 h, cooled to room temperature, and transferred to a sealed flask via a cannular, and treated chloro(chloromethyl)dimethyl silane (5.5 ml, mmol) through syringe. The mixture was heated to reflux overnight. The reaction was quenched by addition 200 ml sat. NH 4 Cl solution and extracted with hexane (50 ml 3). The combined organic layers were dried over Na 2 SO 4, and concentrated under reduced pressure. The crude was purified by distillation under vacuum to provide (chloromethyl)dimethyl((trimethylsilyl)methyl)silane. To a solution of Mg ( mg, mmol) in THF (20 ml) was added (chloromethyl)dimethyl((trimethylsilyl)methyl)silane (6.017 g, mmol in 20 ml THF) 56

70 dropwise. The mixture was heated to reflux for 2 h and transferred to another flask via cannular. Chlorodimethylsilane (5.26 g, mmol) was added dropwise to the flask in an ice bath. The mixture was allowed to warm to room temperature and stirred overnight. The reaction was quenched by addition of sat. NH 4 Cl and extracted with hexane (50 ml 3). The combined organic layers was dried over Na 2 SO 4, and concentrated under reduced pressure to obtain clear oil. The crude was purified by vacuum distillation to afford 7. 1 H NMR (500 MHz, Chloroform-d) δ 0.10 (s, 3H), 0.09 (s, 3H), 0.05 (s, 6H), 0.02 (s, 9H), (s, 1H), (s, 1H), (s, 2H) (Si-H can not be observed due to multiplet coupling). 1-(benzyloxy)-4-(undec-10-enyloxy)benzene 8a To a solution of 4-(benzyloxy)phenol (1.0 g, 5.0 mmol), triphenylphosphine (1.7 g, 6.5 mmol) and 10-Undecen-1-ol (1.46 ml, 7.49 mmol) in THF at room temperature was added diethylazodicarboxylate (DEAD 1.2 ml, 7.49 mmol). The mixture was stirred overnight, quenched by the addition of sodium hydroxide solution (0.5N, 50 ml) and extracted with ether (50 ml 3). The combined organic layers were washed with brine and dried over Na 2 SO 4 and concentrated under reduced pressure. The crude product was purified by column chromatography (EtOAc/Hexane 1:20) to give 8a (1.23 g, 70%). 1 H NMR (500 MHz, Chloroform-d) δ (m, 5H), 6.88 (m, 2H), (m, 2H) (m, 2H), 5.86 (ddt, J = 16.9, 10.2, 6.7 Hz, 1H), 5.07 (dt, J = 2.2, 1.6 Hz, 1H), 5.04 (s, 2H), 4.99 (ddt, 57

71 J = 10.2, 2.2, 1.2 Hz, 1H), 3.93 (t, J = 6.5 Hz, 2H), 2.02 (m, 2H), 1.73 (p, J = 7.0 Hz, 2H), (m, 12 H). 13 C NMR (100 MHz, CDCl , 139.2, 137.3, 128.5, 127.9, 127.5, 115.7, 115.3, 114.1, 70.7, 68.6, 33.8, 29.5, 29.4, 29.4, 29.1, 28.9, In essentially the same manner, 1-(benzyloxy)-4-(dec-9-enyloxy)benzene 8b was prepared in 69% yield. 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), (m, 2H), 5.86 (ddt, J = 16.9, 10.2, 6.7 Hz, 1H), (m, 3H), 4.99 (ddt, J = 10.2, 2.2, 1.2 Hz, 1H), 3.93 (t, J = 6.6 Hz, 2H), (m, 2H), 1.79 (dq, J = 8.4, 6.6 Hz, 2H), (m, 10H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , 70.79, 68.70, 33.99, 29.61, 29.55, 29.25, 29.09, HRMS m/z calcd for C 23 H 30 O 2 Li + : ; found: Componds 8c, 8d, 8e, 8f, 8g were similarly prepared in 75%, 70%, 73%, 75% and 78% yield, respectively. 8c: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), (m, 2H), 5.91 (ddt, J = 16.9, 10.2, 6.7 Hz, 1H), (m, 4H), 3.96 (t, J = 6.5 Hz, 2H), (m, 2H), 1.83 (dq, J = 8.3, 6.6 Hz, 2H), (m, 9H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , 70.74, 68.63, 33.97, 29.54, 29.44, 29.24, 29.03, HRMS m/z calcd for C 44 H 56 O 4 Li + : ; found:

72 8d: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), 6.92 (dd, J = 9.1, 1.7 Hz, 2H), 5.92 (ddtd, J = 16.9, 10.1, 6.7, 1.8 Hz, 1H), (m, 4H), 3.97 (t, J = 6.5 Hz, 2H), 2.17 (q, J = 6.9 Hz, 2H), 1.85 (p, J = 6.8 Hz, 2H), (m, 6H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , 70.68, 68.54, 33.87, 29.47, 29.02, 28.97, HRMS m/z calcd for C 42 H 52 O 4 Li + : ; found: e: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), (m, 2H), 5.87 (ddt, J = 16.9, 10.2, 6.7 Hz, 1H), (m, 4H), 3.94 (t, J = 6.5 Hz, 2H), (m, 2H), (m, 2H), (m, 4H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , 70.79, 68.59, 33.89, 29.40, 28.84, HRMS m/z calcd for C 40 H 48 O 4 Li + : ; found: f: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), (m, 2H), 5.98 (ddt, J = 16.9, 10.2, 6.6 Hz, 1H), (m, 4H), 4.01 (t, J = 6.4 Hz, 2H), 2.26 (m, 2H), (m, 2H), (m, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , 70.58, 68.26, 33.56, 28.91, HRMS m/z calcd for C 38 H 44 O 4 Li + : ; found:

73 8g: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), (m, 2H), 5.92 (ddt, J = 16.9, 10.2, 6.6 Hz, 1H), (m, 4H), 3.97 (t, J = 6.5 Hz, 2H), (m, 2H), (m, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , 70.75, 67.86, 30.30, HRMS m/z calcd for C 36 H 40 O 4 Li + : ; found: Trisilanyl benzene 9a Freshly dry toluene (12 ml) was cannulated to a flask charged with 8a (1.23 g, 3.49 mmol) and 7 (1.14 g, 5.23 mmol). The flask was purged with Ar, treated with Karstedt s catalyst (0.7 ml) dropwise and wrapped with aluminum foil. The mixture was stirred at room temperature for 3 days and concentrated under reduced pressure. The crude product was purified by column chromatography (Hexane/CHCl 3 2:1) to give 9a (1.3 g, 65%). 1 H NMR (500 MHz, Chloroform-d) δ (m, 5H), 6.88 (m, 2H), 6.80 (m, 2H), 4.99 (s, 2H), 3.87 (t, 2H, J = 6.5 Hz), 1.73 (p, 2H, J = 7.0 Hz), (m, 16 H), 0.44 (m, 2H), 0.00 (m, 21H), (m, 4H). 13 C NMR (100 MHz, CDCl 3 ) δ 153.5, 152.8, 137.3, 128.5, 127.9, 127.5, 115.8, 115.4, 70.7, 68.6, 33.7, 29.7, 29.6, 29.6, 29.4, 29.4, 26.1, 23.4, 18.1, 5.8, 4.0, 2.5, 1.5, HRMS m/z calcd for C 33 H 58 O 2 Si 3 H + : ; found:

74 Componds 9b, 9c, 9d, 9e, 9f, 9g were similarly prepared in 65%, 66%, 65%, 68%, 66%, and 65% yield, respectively. 9b: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), (m, 2H), 5.02 (s, 2H), 3.91 (t, J = 6.6 Hz, 2H), 1.76 (m, 2H), 1.45 (m, 2H), (m, 12H), 0.49 (m, 2H), 0.06 (s, 6H), 0.04 (s, 9H), 0.01 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , 70.87, 68.80, 33.95, 29.86, 29.79, 29.66, 29.61, 26.29, 24.20, 18.28, 5.98, 4.21, 2.70, 1.71, HRMS m/z calcd for C 32 H 56 O 2 Si 3 H + : ; found: c: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), (m, 2H), 5.03 (s, 2H), 3.91 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 12H), (m, 2H), 0.07 (s, 6H), 0.05 (s, 9H), 0.02 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , 70.86, 68.78, 33.93, 29.77, 29.69, 29.61, 29.56, 26.29, 24.20, 18.27, 5.98, 4.20, 2.70, 1.71, HRMS m/z calcd for C 31 H 54 O 2 Si 3 H + : ; found: d: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), (m, 2H), 5.03 (s, 2H), 3.92 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 10H), (m, 2H), 0.08 (s, 6H), 0.05 (s, 9H), 0.03 (s, 6H), (s, 2H), (d, J = 1.2 Hz, 2H). 61

75 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , 70.86, 68.78, 33.87, 29.61, 29.54, 26.32, 24.19, 18.28, 5.99, 4.21, 2.71, 1.72, HRMS m/z calcd for C 30 H 52 O 2 Si 3 H + : ; found: e: 1 H NMR (500 MHz, Chloroform-d) δ (m, 5H), (m, 2H), (m, 2H), 5.03 (s, 2H), 3.91 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 8H), (m, 2H), 0.07 (s, 6H), 0.04 (s, 9H), 0.02 (s, 6H), (s, 2H), (s, 2H). HRMS m/z calcd for C 29 H 50 O 2 Si 3 H + : ; found: f: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), 6.94 (dd, J = 9.1, 2.2 Hz, 2H), (dd, J = 9.1, 2.2 Hz, 2H), 5.04 (s, 2H), 3.93 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 6H), (m, 2H), 0.10 (s, 6H), 0.08 (s, 9H), 0.05 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , 70.82, 68.75, 33.65, 29.55, 25.99, 24.14, 18.23, 5.97, 4.19, 2.71, 1.72, HRMS m/z calcd for C 28 H 48 O 2 Si 3 H + : ; found: g: 1 H NMR (400 MHz, Chloroform-d) δ (m, 5H), (m, 2H), 6.87 (m, 2H), 5.04 (s, 2H), 3.93 (t, J = 6.6 Hz, 2H), (m, 2H), 1.51 (m, 2H), (m, 2H), (m, 2H), 0.10 (s, 6H), 0.07 (s, 9H), 0.06 (s, 6H), (s, 2H), (s, 2H). 62

76 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , 70.84, 68.75, 30.24, 29.35, 24.06, 18.24, 5.98, 4.20, 2.71, 1.72, HRMS m/z calcd for C 27 H 46 O 2 Si 3 H + : ; found: Phenol 10a An ethanol solution (20 ml) of 9a (1.3 g, 2.28 mmol) was treated with palladium on charcoal (10%, mg) at room temperature overnight under a hydrogen atmosphere. The reaction mixture was filtered through a short pad of celite and rinsed with EtOAc. Evaporation of the solvent gave 10a (1.04 g, 95%) without further purification. 10a: 1 H NMR (400 MHz, Chloroform-d) δ6.79 (d, J = 9.2 Hz, 2H), 6.75 (d, J = 9.2 Hz, 2H), 5.67 (s, 1H), 3.89 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 16H), 0.50 (m, 2H), 0.07 (s, 6H), 0.04 (s, 9H), 0.02 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , 69.05, 33.93, 29.91, 29.86, 29.75, 29.65, 29.57, 29.54, 26.24, 24.18, 18.25, 5.96, 4.19, 2.70, 1.70, Componds 10b, 10c, 10d, 10e, 10f, 10g were similarly prepared in 95%, 97%, 96%, 98%, 98%, and 99% yield, respectively. 63

77 10b: 1 H NMR (400 MHz, Chloroform-d) δ6.79 (d, J = 9.2 Hz, 2H), 6.75 (d, J = 9.2 Hz, 2H), 5.87 (s, 1H), 3.90 (t, J = 6.3 Hz, 2H), (m, 2H), (m, 14H), 0.50 (m, 2H), 0.07 (s, 6H), 0.05 (s, 9H), 0.02 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , 69.05, 33.93, 29.84, 29.77, 29.63, 29.59, 29.55, 26.24, 24.18, 18.25, 5.95, 4.18, 2.69, 1.70, c: 1 H NMR (400 MHz, Chloroform-d) δ6.79 (d, J = 9.2 Hz, 2H), 6.75 (d, J = 9.2 Hz, 2H), 5.64 (s, 1H), 3.90 (t, J = 6.6 Hz, 2H), (m, 2H), 1.38 (m, 12H), 0.50 (m, 2H), 0.07 (s, 6H), 0.04 (s, 9H), 0.02 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , 69.05, 33.91, 29.75, 29.67, 29.57, 29.54, 26.24, 24.18, 18.25, 5.96, 4.19, 2.70, 1.70, d: 1 H NMR (400 MHz, Chloroform-d) δ 6.79 (d, J = 9.2 Hz, 2H), 6.75 (d, J = 9.2 Hz, 2H), 4.74 (s, 1H), 3.89 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 10H), (m, 2H), 0.04 (s, 6H), 0.02 (s, 9H), (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , 68.95, 33.86, 29.59, 29.53, 26.31, 24.18, 18.27, 5.98, 4.21, 2.70, 1.70, e: 1 H NMR (400 MHz, Chloroform-d) δ 6.79 (d, J = 9.4 Hz, 2H), 6.75 (d, J = 9.4 Hz, 2H), 4.83 (s, 1H), 3.90 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 8H), (m, 2H), 0.05 (s, 6H), 0.03 (s, 9H), 0.00 (s, 6H), (s, 2H), (s, 2H). 64

78 13 C NMR (101 MHz, CDCl 3 ) δ , , , , 68.97, 33.84, 29.61, 29.36, 26.21, 24.14, 18.26, 5.98, 4.20, 2.70, 1.71, f: 1 H NMR (500 MHz, Chloroform-d) δ6.79 (d, J = 9.4 Hz, 2H), 6.75 (d, J = 9.4 Hz, 2H), 3.88 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 6H), (m, 2H), 0.05 (s, 6H), 0.03 (s, 9H), 0.00 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , 69.06, 33.64, 29.50, 25.96, 24.12, 18.22, 5.96, 4.18, 2.70, 1.71, g: 1 H NMR (400 MHz, Chloroform-d) δ 6.79 (d, J = 9.4 Hz, 1H), 6.75 (d, J = 9.3 Hz, 1H), 4.92 (s, 1H), 3.90 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), 0.06 (s, 6H), 0.03 (s, 9H), 0.01 (s,6h), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , 69.00, 30.21, 29.31, 24.04, 18.24, 5.97, 4.19, 2.70, 1.71, Benzoate 1(11) 10a (504.1 mg, 1.07 mmol), 6 (374.5 mg, 1.07 mmol) and DMAP (13.1 mg) were charged with a 50 ml round bottom flask. Dry DCM (10 ml) was added, followed by DCC (264.9 mg,

79 mmol). The reaction was stirred at room temperature for 1 day, diluted with DCM, quenched by addition of 5% HCl (50 ml) and extracted with DCM (50 ml 3). The combined organic layers were washed with brine and dried over Na 2 SO 4 and concentrated under reduced pressure. The crude product was purified by column chromatography (EtOAc/Hexane 3:97) to give 1(11) (767.3 mg, 90%). The final product was purified further via recrystallization with CH 3 CN. 1 H NMR (400 MHz, Chloroform-d) δ (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), 4.40 (h, J = 6.1 Hz, 1H), 3.96 (t, J = 6.5 Hz, 2H), (m, 4H), (m, 28H), (m, 2H), (m, 2H), 0.07 (s, 6H), 0.05 (s, 9H), 0.02 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , , , , 93.44, 87.57, 74.11, 68.56, 36.58, 33.94, 31.99, 29.87, 29.83, 29.81, 29.64, 29.62, 29.49, 29.46, 26.26, 25.67, 24.19, 22.81, 19.86, 18.25, 14.31, 5.96, 4.19, 2.69, 1.70, HRMS m/z calcd for C 49 H 76 O 4 Si 3 Li + : ; found: Componds 1(10), 1(9), 1(8), 1(7), 1(6), 1(5) were similarly prepared in 92%, 90%, 89%, 93%, 90%, and 91% yield, respectively. 1(10): 1 H NMR (400 MHz, Chloroform-d) δ 8.16 (d, J = 8.7 Hz, 2H), 7.62 (d, J = 8.7 Hz, 2H), 7.48 (d, J = 8.9 Hz, 2H), 7.12 (d, J = 9.0 Hz, 2H), 6.93 (d, J = 9.1 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H), 4.40 (h, J = 6.1 Hz, 1H), 3.96 (t, J = 6.5 Hz, 2H), (m, 2H), (m, 2H), (m, 26H), (m, 2H), (m, 2H), 0.06 (s, 6H), 0.03 (s, 9H), 0.01 (s, 6H), (s, 2H), (s, 2H). 66

80 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , , , , 93.44, 87.57, 74.17, 68.61, 36.59, 33.95, 32.00, 29.85, 29.79, 29.63, 29.61, 29.49, 29.47, 26.27, 25.69, 24.20, 22.82, 19.87, 18.27, 14.32, 5.97, 4.20, 2.69, 1.70, HRMS m/z calcd for C 48 H 74 O 4 Si 3 Li + : ; found: (9): 1 H NMR (400 MHz, Chloroform-d) δ 8.16 (d, J = 8.7 Hz, 2H), 7.62 (d, J = 8.7 Hz, 2H), 7.48 (d, J = 8.8 Hz, 2H), 7.12 (d, J = 9.0 Hz, 2H), 6.93 (d, J = 9.0 Hz, 2H), 6.87 (d, J = 8.8 Hz, 2H), 4.40 (h, J = 6.1 Hz, 1H), 3.96 (t, J = 6.5 Hz, 2H), (m, 2H), (m, 2H), (m, 24H), (m, 2H), (m, 2H), 0.05 (s, 6H), 0.03 (s, 9H), 0.00 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , , , , 93.44, 87.57, 74.18, 68.62, 36.59, 33.93, 32.00, 29.77, 29.67, 29.56, 29.50, 29.47, 26.26, 25.69, 24.20, 22.82, 19.88, 18.27, 14.32, 5.97, 4.20, 2.69, 1.70, HRMS m/z calcd for C 47 H 72 O 4 Si 3 NH 4 + : ; found: (8): 1 H NMR (400 MHz, Chloroform-d) δ (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), 4.40 (h, J = 6.1 Hz, 1H), 3.96 (t, J = 6.5 Hz, 2H), (m, 2H), (m, 2H), (m, 22H), (m, 2H), (m, 2H), 0.05 (s, 6H), 0.03 (s, 9H), 0.01 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , , , , 93.43, 87.57, 74.17, 68.62, 36.59, 33.87, 32.00, 29.58, 67

81 29.54, 29.49, 29.47, 26.30, 25.69, 24.19, 22.82, 19.88, 18.27, 14.32, 5.98, 4.20, 2.70, 1.70, HRMS m/z calcd for C 46 H 70 O 4 Si 3 H + : ; found: (7): 1 H NMR (500 MHz, Chloroform-d) δ (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), 4.40 (h, J = 6.1 Hz, 1H), 3.96 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 2H), (m, 20H), (m, 2H), (m, 2H), 0.05 (s, 6H), 0.03 (s, 9H), 0.00 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , , , , 93.44, 87.57, 74.18, 68.61, 36.59, 33.84, 32.00, 29.52, 29.47, 29.35, 26.21, 25.69, 24.15, 22.82, 19.89, 18.27, 14.32, 5.99, 4.21, 2.70, 1.70, HRMS m/z calcd for C 45 H 68 O 4 Si 3 H + : ; found: (6): 1 H NMR (400 MHz, Chloroform-d) δ (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), 4.40 (h, J = 6.1 Hz, 1H), 3.96 (t, J = 6.5 Hz, 2H), (m, 2H), (m, 2H), (m, 18H), (m, 2H), (m, 2H), 0.06 (s, 6H), 0.04 (s, 9H), 0.02 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , , , , 93.44, 87.57, 74.15, 68.61, 36.58, 33.63, 32.00, 29.47, 29.43, 25.97, 25.68, 24.14, 22.82, 19.87, 18.23, 14.32, 5.97, 4.19, 2.70, 1.70, HRMS m/z calcd for C 44 H 66 O 4 Si 3 Li + : ; found:

82 1(5): 1 H NMR (400 MHz, Chloroform-d) δ (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), (m, 2H), 4.40 (h, J = 6.1 Hz, 1H), 3.96 (t, J = 6.6 Hz, 2H), (m, 2H), (m, 2H), (m, 16H), (m, 2H), (m, 2H), 0.06 (s, 6H), 0.03 (s, 9H), 0.02 (s, 6H), (s, 2H), (s, 2H). 13 C NMR (101 MHz, CDCl 3 ) δ , , , , , , , , , , , , , 93.43, 87.57, 74.18, 68.62, 36.59, 32.00, 30.22, 29.47, 29.23, 25.69, 24.05, 22.82, 19.88, 18.25, 14.32, 5.98, 4.20, 2.70, 1.70, HRMS m/z calcd for C 43 H 64 O 4 Si 3 H + : ; found: References [1] K. D havé, P. Rudquist, S. T. Lagerwall, H. Pauwels, W. Drzewinski, R. Dabrowski, Appl. Phys. Lett. 2000, 76, [2] B. S. Lagerwall, A. Dahlgren, P. Jägemalm, P. Rudquist, D. Koen, H. Pauwels, R. Dabrowski, W. Drzewinski, Adv. Funct. Mater. 2001, 11, 87. [3] K. D havé, A. Dahlgren, P. Rudquist, J. P. F. Lagerwall, G. Andersson, M. Matuszczyk, S. T. Lagerwall, R. Dabrowski, W. Drzewinski, Ferroelectrics 2000, 244, 115. [4] R. Dąbrowski, J. Gąsowska, J. Otón, W. Piecek, J. Przedmojski, M. Tykarska, Displays 2004, 25, 9. [5] P. Nayek, S. Ghosh, S. Kundu, S. K. Roy, T. P. Majumder, N. Bennis, J. M. Otón, R. Dabrowski, J. Phys. D Appl. Phys. 2009, 42, [6] P. Rudquist, Liq. Cryst. 2013, DOI: / [7] R. Dąbrowski, P. Kula, Z. Raszewski, W. Piecek, J. M. Otón, A. Spadło, Ferroelectrics 2010, 395, 116. [8] (a) C. Tschierske, J. Mater. Chem. 2001, 11, 2647; (b) M. Glaser, N. Clark, Phys. Rev. E 2002, 66, ; (c) P. Maiti, Y. Lansac, M. Glaser, N. Clark, Phys. Rev. Lett. 2002, 88, 69

83 065504; (d) P. H. J. Kouwer, G. H. Mehl, Angew. Chem. Int. Ed. 2003, 42, 6015; (e) S. Yazaki, M. Funahashi, J. Kagimoto, H. Ohno, T. Kato, J. Am. Chem. Soc. 2010, 132, 7702; (f) C. Tschierske, Angew. Chem. Int. Ed. 2013, 52, 8828; (g) H. K. Bisoyi, V. A. Raghunathan, S. Kumar, Chem. Commun. 2009, [9] P. Maiti, Y. Lansac, M. Glaser, N. Clark, Phys. Rev. Lett. 2002, 88, [10] J. Wu, W. Pisula, K. Müllen, Chem. Rev. 2007, 107, 718. [11] A. D. L. Chandani, T. Hagiwara, Y. Suzuki, Y. Ouchi, H. Takezoe, A. Fukuda, Jan. J. Appl. Phys. 1988, 27, L729. [12] A. D. L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe, A. Fukuda, Jan. J. Appl. Phys. 1989, 28, L1265. [13] A. de Vries, Mol. Cryst. Liq. Cryst. Lett., 1977, 41, 27. [14] Y. Takanishi, A. Ikeda, H. Takezoe, A. Fukuda, Phys. Rev. E 1995, 51, 400. [15] K. D havé, P. Rudquist, S. T. Lagerwall, H. Pauwels, W. Drzewinski, R. Dabrowski, Appl. Phys. Lett. 2000, 76, [16] M. Glaser, N. Clark, Phys. Rev. E 2002, 66, [17] K. Ito, K. Endo, K. Hori, T. Nemoto, H. Uekusa, Y. Ohashi, Liq. Cryst. 1994, 17, 747. [18] (a) F. Tournilhac, L. M. Blinov, J. Simon, S. V. Yablonsky, Nature 1992, 359, 621; (b) C. Keith, R. A. Reddy, A. Hauser, U. Baumeister, C. Tschierske, J. Am. Chem. Soc. 2006, 128, 3051; (c) G. Ungar, C. Tschierske, V. Abetz, R. Holyst, M. A. Bates, F. Liu, M. Prehm, R. Kieffer, X. Zeng, M. Walker, B. Glettner, A. Zywocinski, Adv. Funct. Mater. 2011, 21, [19] (a) C. Tschierske, Angew. Chem. Int. Ed. 2013, 52, 8828; (b) G. Ungar, C. Tschierske, V. Abetz, R. Holyst, M. a. Bates, F. Liu, M. Prehm, R. Kieffer, X. Zeng, M. Walker, B. Glettner, A. Zywocinski, Adv. Funct. Mater. 2011, 21, [20] (a) C. Müller, L. J. Ackerman, J. N. H. Reek, P. C. J. Kamer, P. W. N. M. Van Leeuwen, J. Am. Chem. Soc. 2004, 126, 14960; (b) M. Hird, Chem. Soc. Rev. 2007, 36, 2070; (c) L. Liu, Y. Zang, S. Hadano, T. Aoki, M. Teraguchi, T. Kaneko, T. Namikoshi, Macromolecules 2010, 43, 9268; (d) J. Mei, D. H. Kim, A. L. Ayzner, M. F. Toney, Z. Bao, J. Am. Chem. Soc. 2011, 133, [21] G. Dantlgraber, A. Eremin, S. Diele, A. Hauser, H. Kresse, G. Pelzl, C. Tschierske, Angew. Chem. Int. ed. 2002, 41, [22] S. Prasad, D. Rao, S. Sridevi, C. Lobo, B. Ratna, J. Naciri, R. Shashidhar, Phys. Rev. Lett. 2009, 102,

84 [23] C. Keith, R. A. Reddy, U. Baumeister, H. Hahn, H. Lang, C. Tschierske, J. Mater. Chem. 2006, 16, [24] R. A. Reddy, C. Zhu, R. Shao, E. Korblova, T. Gong, Y. Shen, E. Garcia, M. A. Glaser, J. E. Maclennan, D. M. Walba, N. A. Clark, Science 2011, 332, 72. [25] S. J. Cowling, A. W. Hall, J. W. Goodby, Y. Wang, H. F. Gleeson, J. Mater. Chem. 2006, 16, [26] S. K. Lee, S. Heo, J. G. Lee, K.-T. Kang, K. Kumazawa, K. Nishida, Y. Shimbo, Y. Takanishi, J. Watanabe, T. Doi, T. Takahashi, H. Takezoe, J. Am. Chem. Soc. 2005, 127, [27] D. Link, J. Maclennan, N. Clark, Phys. Rev. Lett. 1996, 77, [28] A. D. L. Chandani, T. Hagiwara, Y. Suzuki, Y. Ouchi, H. Takezoe, A. Fukuda, Jpn. J. Appl. Phys. 1988, 27, L729. [29] M. Żurowska, R. Dąbrowski, J. Dziaduszek, W. Rejmer, K. Czupryński, Z. Raszewski, W. Piecek, Mol. Cryst. Liq. Cryst. 2010, 525, 219. [30] A. A. Sigarev, J. K. Vij, A. W. Hall, S. Cowling, J. W. Goodby, Ferroelectrics 2006, 343, 167. [31] P. Schiller, G. Pelzl, D. Demus, Liq. Cryst. 1987, 2, 21. [32] Y. Takanishi, a Ikeda, H. Takezoe, a Fukuda, Phys. Rev. E. 1995, 51, 400. [33] The molecular length was obtained by Spartan program using a density functional model (B3LYP 631G*). The molecular length was calculated assuming that an overall more straight conformation with a gauche bend at OC-CC bond on both sides of the aromatic core shown in Figure [34] D. M. Walba, Ferroelectric Liquid Crystals, 1991, vol. 1. [35] K. Taguchi, K. Arakawa, T. Eguchi, K. Kakinuma, Y. Nakatani, G. Ourisson, New J. Chem. 1998, 22,

85 3 An Electric-Field-Responsive Discotic Liquid-Crystalline Hexaperi-hexabenzocoronene/Oligothiophene Hybrid 3.1 Introduction Organic electronics (plastic electronics) that utilize conjugated polymers and conductive small molecules as active components in electronic devices, 1 have increasingly become a field of intense study because of the prospect of the creation of a new industry. In contrast to traditional electronics, that use inorganic conductors and semiconductors, such as copper and silicon, respectively, conjugated organic molecules are lighter, more flexible, and less expensive than inorganic conductors. Thus the conjugated organic molecules are ideal candidates for optical and electronic devices such as light-emitting diodes, photovoltaic diodes (PVD), field-effect transistors, photocopiers, laser printers, and holographic data storage. However, the conventional conjugated materials, i.e. linear oligomers and polymers show a serious limitation due to the onedimensional nature of their structures. This creates a need for new conjugated molecules with innovative design and semiconducting behavior. Among the various new materials developed for organic electronics, conjugated LCs are currently regarded as a new generation of organic semiconductors due to their long-range self-assembly (order) and self-healing (dynamics). 2 Order is possibly the most important parameter that dominates the performance of organic semiconductors in devices. Conjugated LCs provide the decisive advantage of controlling order in bulk and at all length scales from molecular to macroscopic distances (see Figure 3.1). Due to their liquid-like character, the ability of conjugated LCs to self-heal structural defects such as grain boundaries, and consequently achievement of several square millimeters of large single 72

86 domains with several micrometers film thickness, are the most striking examples of the effect of dynamics. 3 Figure 3.1 Typical length scales encountered in organic electronics and control of order achievable with LC semiconductors. 3 Among conjugated LCs, the calamitic (rod-like) LCs possessing one-dimensional molecular structures tend to self-assemble into smectic mesophases, which exhibit a twodimensional charge transport comparable to that observed for the herringbone packing of pentacene and oligothiophenes. 4 Thus the extent of frontier orbital overlap is rather moderate. Discotic LCs show one-dimensional charge transport within the columns with typical core-core distance of about 3.5 Å. The orbital overlap is much larger than that of calamitics (Figure 3.2). 73

87 Figure 3.2 Schematic illustration of calamitic (left) and discotic (right) semiconductors. 3 Polycyclic aromatic hydrocarbons (PAHs) 5 can be regarded as two-dimensional graphite segments composed of all sp 2 carbons and are potential materials for organic electronics 6 since their particular electronic properties. Typical examples of disc-like PAHs with high purity are triphenylene and hexa-peri-hexabenzocoronene (HBC) derivatives (Figure 3.3). The synthesis of HBC derivatives in the nineties 7 can be regarded as a significant achievement not only because of the high charge carrier mobility but also because it has paved the way to large columnar liquid crystalline (COL LC) PAHs. 8,9 The flexible peripheral substituents that are typically flexible aliphatic chains around the aromatic core, provide opportunities to control the solubility and thermal behavior, which are essential for the processing of materials. Moreover, introduction of flexible chains increases the disorder and leads to a nanophase segregation between the highly ordered aromatic core stacking due to core-core van der Walls attraction, and the disordered pendant chains filling space. Values up to 1.1 cm 2 V -1 S -1 for HBC derivatives have been detected by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) measurement. 10 HBCs are qualified as active semiconductors in organic field-effect transistors (FETs) and photovoltaic devices. 6b,11,12 74

88 Figure 3.3 Selected examples of widely studied discotic HBC derivatives. The one-dimensional charge carrier transport along the columnar axis is ensured by the pronounced intracolumnar stacking and the effective filling of the insulating aliphatic chains between the columns. However such one-dimensional charge transport is rather sensitive to structural defects. In thin-film devices, due to the disorder of the active component deposited between the electrodes, grain boundaries (defect), and the metal-interface resistance, the charge transport in bulk is usually limited. Therefore, control of macroscopic orientations of columnar superstructures is a key issue to obtain optimized performance for electronic devices with different geometries. For FETs, an edge-on orientation of discotics in uniaxial parallel alignment is required, so that charge carriers transport through the columns from the source electrode to the drain electrode under controlled gate voltage (Figure 3.4). For photovoltaic cells, homeotropic alignment with a face-on orientation of the discs allows faster charge transport between the top and bottom electrode to enhance the photovoltaic performance. 75

89 Figure 3.4 Schematic illustration of two types of electronic devices and their desired supramolecular arrangements on surfaces with edge-on orientation of the molecules for FETs versus face-on arrangement for photovoltaic devices. 13 Uniform parallel alignment of HBC derivertives in thin films has been achieved by serveral methods, including self-assembly from solution on friction-deposited polytetrafluoroethylene (PTFE), 14 a multi-monolayer Langmuir-Blodgett (LB) technique, 15 zone casting, 16 and application of a magnetic field 17 or electric field 18 to a drop-cast solution. The LB techniques 19 are well-established alignment methods for forming well-defined mono- or multi-layers. Amphiphilic HBC molecules 20 spread from the solution at the air-water interface, and compress to monolayers, which can be then transferred to a solid substrate. 76

90 Figure 3.5 Schematic illustration of the deposition of the amphiphilic HBC molecules onto a substate with the edge-on orientation using the LB technique. 7b The zone-casting technique is simple and effecient method for fabriction of highly ordered layers of HBC derivatives over several square centimeters with a thickness of about 15 nm, such as HBC-C12 and HBC-PhC12. Deposition of an HBC solution is from a nozzle onto a moving substate. A concentration gradient is formed between the nozzle and the moving support. At the critical concentration, the material is nucleated from the solution onto the support as an aligned thin film (Figure 3.6). Figure 3.6 Schematic illustration of zone-casting HBC-C12. 6b 77

91 Highly ordered and uniaxially planar-aligned PAHs thin films, including HBCs and triphenylenes, can be fabricated by crystallization from solution onto preoriented and frictiondeposited PTFE (Figure 3.7). Figure 3.7 Schematic representation of the columnar HBC stacking with uniaxially parallel alignment. 6b The methods for uniformly homeotropic alignment are not well-established. As for HBCs, a spontaneous formation of a homeotropic phase is observed in HBC with an oxygen atom (Figure 3.8a) 21 on the pendant side chains even at very rapid cooling rates, and by slow cooling of thin isotropic films of HBC bearing a tethered anthraquinone (AQ) unit or dimethoxyanthracene (DMA) unit on the side chain (Figure 3.8b) 22 between the two glass slides. 78

92 Figure 3.8 Chemical structures of HBC derivatives with spontaneous homeotropic alignment. 3.2 Goals Columnar liquid crystals (COL LCs) formed from discotic mesogens are promising materials for a number of applications in organic electronics 6, due to their advantageous properties, such as macroscopic self-assembly, self-healing, and ease of processing. Discotic materials possessing the hexa-peri-hexabenzocoronene (HBC) core show the highest charge carrier mobility observed to date in COL LCs. 23 Discotic mesogens, including HBC derivatives, 79

93 are typically composed of more or less rigid planar aromatic cores with flexible chains laterally attached to the cores. Exploration for new COL LC materials enhances the molecular structural diversity achievable, and can allow tuning of physical properties of COL phases at the molecular and supramolecular levels. As for all COL LCs, realizing the full potential for applications requires achieving high quality alignment (uniform macroscopic orientation of the LC). 2,6,24 For example, HBC-based columnar structures show high charge-transport anisotropy in well aligned films. 17,25 If the director (column axis) is parallel to the substrates, the system is attractive for fabrication of field-effect transistors (FETs), while homeotropic alignment (director normal to the substrate surface) is well suited for photovoltaic cells and light-emitting diodes. 6 Uniform parallel alignment of HBCs in thin films has been achieved by several methods, including selfassembly from solution on friction-deposited polytetrafluoroethylene (PTFE), 14 a multimonolayer Langmuir-Blodgett technique, 15 zone casting, 16 and application of a magnetic field 17 or electric field to a drop-cast solution. 18 None of these methods involves alignment of a sample in LC phase by applying an electric field; all require solution processing. In addition, the high clearing temperatures of most HBC mesophases (above 300 C) 22,26 might be limiting with regard to thermal processing of the materials through the isotropic phase. Homeotropic alignment seems to be the most elusive organization for high molecular-weight mesogens, such as those with the HBC core. Very little success in obtaining high quality homeotropic alignment has been reported to date. 27 Furthermore, the examples of COL LCs responsive to electric fields are rare. With a few interesting exceptions, 28 most reported COL LC materials showing a response to an applied electric field in several micrometers thickness involve hydrogen-bonding motifs serving as a responsive handle. 29 For example, Takuzo Aida 29e et al. reported the COL LCs with amide units on side chains attached to the various aromatic cores can be reoriented by 80

94 an electric field, resulting in unidirectional homeotropic or parallel alignment over a large area. The resultant macroscopic columnar orientation was maintained even after the electric field was switched off (Figure 3.9). Figure 3.9 Structures and phase diagrams of COL LCs that are responsive to electric fields and their columnar orientation is maintained even after the E field is switched off. Phase-transition temperatures are in C. Symbols Cr, Col h, and Iso denote crystalline, hexagonal columnar, and isotropic phases, respectively. Values in parentheses below the symbol Col are intercolumnar distances in nm, and the subscripted values are the observed temperatures in C. 29e Development of new HBC-based materials by tuning peripheral functionalities to enable high quality alignment of mesophases with several µm thickness is highly attractive. And, this is most useful if alignment can be achieved by well-known techniques typically used for nematics and smectics. 81

95 3.3 Design and Synthesis Since thiophene-based materials can be easily functionalized, 30 have been extensively utilized for organic electronics, 31 and can self-assemble into LCs, 32 a HBC derivative with six covalently tethered oligothiophene units was designed. The click reaction, Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of azides 33 was designed as a means of covalently tethering six oligothiophene units to the HBC core. Initially quaterthiophene without hexyl substitution was chosen to be attached to the HBC core because smectic and nematic LC phases were observed in such rigid quaterthiophenes. 30 The combination of these rigid quaterthiophene units with the HBC core (Figure 3.10) could be more likely to self-assemble into a columnar structure. 82

96 Figure 3.10 Retro synthesis of a HBC/ quaterthiophene hybrid HBC-4Th. The synthesis of ethynyl-substituted quaterthiophene is given in Scheme 3.1. The terthiophene is selectively brominated with NBC at the -position and subsequently iodination 83

97 with NIS at the other -position. Sonogshira-type selective coupling of the resulting quaterthiophene with the ethynyltrimethylsilane gave the ethynyl-terthiophene. It should be pointed out that the reaction needed to be conducted at low temperature since bromine can be replaced by ethynyl unit as well. The quaterthiophene was prepared via stille coupling of the ethynyl-terthiophene with ethynyltrimethylsilane. Subsequent desilylation with potassium carbonate yielded conjugated ethynyl functionalized quaterthiophene. Tetra-n-butylammonium fluoride (TBAF) that made the compound decompose cannot be used for desilylation. Scheme 3.1 Synthesis of ethynyl-substituted quaterthiophene. Covalent coupling of the ethynyl-quaterthiophene with a mixture of HBC with azide and chloro-alkyl chains (approximately 2:1 N 3 /Cl) (the synthesis of substituted HBC compounds will be described in Scheme 3.3) afforded a mixture containing some amount of the target HBC- 6QTh. The incompletely substituted product with the chlorine substituents was generated and 84

98 cannot to be purified by recrystallization or column chromatography. So preparation of HBC with hexa-azide is an issue to synthesize the HBC derivative with six covalently tethered alkyloligothiophene units. The mixture exhibited very poor solubility in organic solvents, such as chloroform, toluene, pyridine, tetrahydrofuran. This hampered NMR spectroscopic characterization and purification. With respect to the insolubility issue, I decided introduction of the hexyl chain to the thiophene units at their positions. Scheme 3.2 Tentative synthesis of HBC-6QTh. Herein, a new HBC-based COL LC (1) with six pendant quadra-3-hexylthiophene units attached to the core through long alkyl chains (Figure 3.11) was synthesized. 85

99 1 Figure 3.11 Molecular structure of mesogen 1. The synthesis of COL LC mesogen 1, outlined in Scheme 1, is remarkably efficient, taking advantage of the prototype click reaction, Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of azides 33 with terminal alkynes, to accomplish covalent tethering of six quadra 3 hexylthiophene units to a functionalized HBC core in one step. The selective bromination of commercial dithiophene provided mono-bromide 2. The selective deprotonation of dithiophene and nucleophilic substitution afforded mono-substituted stannane intermediate. The 86

100 quaterthiophene 3 was prepared by selective bromo functionalization of bithiophene 2, which can be allowed to react with stannane intermediated via still coupling. Then selective iodination of the compound 3 afforded iodo mono-substituted quaterthiophene 4, which was then ready for the introduction of ethynyltrimethylsilane via Sonogashira coupling reaction to give 5 in high yield. Subsequent desilylation with potassium carbonate yielded 6 with conjugated ethynyl function. Negishi coupling of (11-methoxyl-11-oxoundecyl) zinc bromide freshly prepared with commercial trimethylsilyl benzene provided 7. The trimethylsilane was converted into iodo function using iodine monochloride, which can be allowed to couple with ethynyltrimethlsilane via Sonogashira reaction. Desilylation with TBAF gave 10 with an unprotected ethynyl function. After coupling with 8 via Sonogashira reaction, the resulting precursor diphenylacetylene 11 was submitted to the Vollhardt Co-catalyzed cyclotrimerization condition 34 for preparing the esterterminated hexa-alkylphenylbenzene 12. The ester groups of compound 12 were reduced to alcohols with LiAlH 4 in THF to provide hexa-alcohol 13, which was then converted to hexabromide 14 in high yield via the Appel reaction. Oxidative cyclization of the hexaphenylbenzene 14 under conditions reported by Müllen (FeCl 3 /MeNO 2 in CH 2 Cl 2 ) afforded the substituted HBC 15. The difference of the reaction from traditional oxidative cyclization was that some bromo substituents were converted to chloro substituents, resulting in 15 possessing a mixture of bromo- and chloro-alkyl chains (approximately 2:1 Br/Cl ratio). Later, it was found that the alkyl chlorides could not be fully converted to azides by reacting with NaN 3 because a lower reactivity of the chloro function. Therefore, both the bromine and chlorine substituents were completely converted to the more reactive iodides by subjecting HBC 15 to excess NaI in dry acetone to afford the hexa-iododide 16, which was then converted to the desired hexa-azide 17 in quantitative yield. Finally, the target HBC-oligothiophene 1 was obtained by coupling hexa- 87

101 azide 17 with ethynyl 3-hexyl-quadrathiophene 6 under typical Cu(I)-catalyzed click conditions in 95% yield after purification. The 1 H NMR (Figure 3.12), 13 C NMR (Figure 3.13 and Figure 3.14) and accurate MS (m/z calcd for C 360 H 492 N 18 S 24 H 2+ 2 : ; found: ) verified the structure and the purity of the compound 1. 88

102 89

103 Scheme 3.3 Synthesis of HBC-hexa-3-hexyl-quadrathiophene 1. Figure H NMR (400 MHz) of 1 in CDCl 3 at 25 C. Figure C NMR (101 MHz) of 1 in CDCl 3 at 25 C. 90

104 Figure C NMR (101 MHz) of 1 above 100 ppm in CDCl 3 at 25 C. The normalized UV-Vis absorptions of 1, the HBC core 15, and the oligothiophene arm 5 in chloroform were shown in Figure The absorption spectrum of 15 show multiple peaks from 350 nm to 450 nm, which are characteristic of the HBC core. 35,36 A large peak around 400 nm for the absorption of 5 is assigned to the - * transition of oligothiophene. 37 The UV-Vis absorption spectrum of 1 keeps the spectral features of the HBC core and exhibits the absorption onset that is same as that of the oligothiophene arm. Therefore, there is no electronic communication between the core and the arms since they are not conjugated. 91

105 Figure 3.15 Normalized UV-vis measurements in chloroform for 1, the oligothiophene arm, and the HBC core. 3.4 Results and Discussion Initial characterization of the compound 1 was accomplished by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The transition temperatures and enthalpies determined by DSC are given in Figure The material forms a thermodynamically stable enantiotropic COL LC phase. The transitions on heating from the crystal to COL LC phase and COL LC to isotropic phase, are first-order (transition enthalpies are 77.2 and 6 kj mol -1, respectively), while on cooling, crystallization occurs well below room temperature (though the enthalpy values suggest that some crystallization occurs before the bulk of the sample crystallizes). The clearing point T i (106.2 C) is 293 C lower than that of the HBC mesogen with six dodecyl pendent groups (399 C). 38 Studies of the phase for mesogen 1 by POM (Figure 3.17a) show a smooth focal conic texture on cooling from the isotropic to the COL LC phase, with extinction brushes oriented parallel and perpendicular to the polarizers, indicating non-uniform parallel alignment of an untilted COL phase. 92

106 The topography of fracture surfaces obtained from LC cells of HBC mesogen 1 quenched in the COL phase and imaged by freeze fracture transmission electron microscopy (FFTEM), are fully consistent with a parallel-aligned hexagonal COL phase, with the inter-column spacing easily resolved (Figure 3.17b, c, d). Here, we find that 1 self-assembles into long columns with diameter of 5 nm in edge-on orientation. Fracture planes follow the interfaces between columns, and in some places across columns, which provides direct evidence for the 2D periodic stack of compound 1 in the COL LC phase. Figure 3.16 DSC traces of 1 on second heating/cooling at a scan rate of 2 C min

107 Figure 3.17 (a) Texture observed for the Col h phase of mesogen 1 at 80 C (Red scale bar represents 100 µm). (b) FFTEM image of a Pt-C replica showing the topography of mesogen 1 quenched from the Col h phase (at T = 80 C) and fractured in the bulk, magnified in (c). (d) Simplified model of the structure observed in (c). (e) Synchrotron small angle scattering observed for a powder sample of 1, and (f) an azimuthal scan of the wide angle scattering obtained from a 2D diffraction pattern for a powder sample of 1 at 84 C. The original 2D wide angle data is given in (g). The hexagonal arrangement of the columns is substantiated by small-angle X-ray diffraction (XRD) experiments. The sequence of equatorial reflection peaks (Figure 3.17e) with d-spacings of 45.7 Å (100), 26.4 Å (110), 22.8 Å (200), 17.2 Å (210), and 15.2 Å (300) and a reciprocal d-spacing ratio of 1: :2: :3, characteristic of a hexagonal lattice of columns for an unoriented powder sample in the COL LC phase. The observed intercolumnar distance of the 94

108 hexagonal lattice of 5.27 nm is shorter than the molecular length in a fully extended conformation (approximately 7 nm), inferring the side chains interpenetrate between neighboring columns, or that disks and/or side chains randomly tilt relative to column axis based on wideangle XRD. Wide-angle XRD (Figure 3.17f, g), shows a broad halo consisting of two reflections centered at about 3.53 Å, a reasonable value of the periodic stacking of the HBC cores within columns, and 4.47 Å, respectively. We suggest that the latter is associated to the liquid-like correlation of the pentacyclic triazole-oligo-3-hexylthiophene units, which is quite similar to stacking distances (~ 4.5 Å) of other oligothiophenes in LC phases 32b,39 and seems reasonable. Since the two wide-angle reflections at 3.53 Å and 4.47 Å, both the HBC core and terminal pentacyclic aromatic units are mainly located in a plane normal to the column axis. A key advantage of LC materials in general is the strong response to application of electric fields. The electro-optic behavior of the COL LC phase of mesogen 1 is particularly interesting in this regard. Specifically, the columns in the non-uniform parallel-aligned samples of the COL LC phase between ITO-coated glass plates, obtained directly by cooling from the isotropic phase, can be easily re-oriented to uniformly homeotropic alignment by application of an electric field along the cell normal. After switching off the field, this uniform alignment is maintained, and can only be changed by heating the sample above the clearing temperature and cooling again without an electric field. This behavior is seen in clean ITO-glass cells, and in cells with spin-coated alignment layers, including nylon (Du Pont Elvamide) and polyimide. For example, COL LC mesogen 1 was filled by capillary action into the commercial ITO/glass LC cell possessing low pre-tilt rubbed polyimide alignment layers, with a cell gap of 4 m. Upon cooling from the isotropic to the COL LC phase, POM under crossed polarizers exhibits the typical birefringent focal conic 95

109 texture seen for COL phases, where the rubbed polyimide gives parallel alignment with no azimuthal orientation. Application of a 2.0 Hz square-wave AC electric field of about ± 30 V/µm at 80 C provided an extremely high quality dark state (Figure 3.18, Figure 3.20a) within 1 min. This result is fully consistent with clean homeotropic alignment of a hexagonal COL LC. The switching voltage threshold for driving the reorientation of a parallel sample to homeotropic as a function of temperature is given in Figure To confirm the conclusions of the POM studies, small-angle X-ray scattering (SAXS) experiments were accomplished using a LC cell prepared using thin glass substrates patterned with an ITO square surrounded by clean glass, where the native parallel alignment (off the electrode surface) could be compared directly to the vertical E- field driven homeotropic alignment (over the ITO electrodes). As shown in Figure 3.20c, the non-ito area exhibits distinct circular reflection, indicating that the columns are randomly arranged with the column axes parallel to the surface (perpendicular to the k-vector of the beam). In contrast, the area between the ITO electrodes, after application of an AC field (± 20 V/ m) 10 days prior to taking the SAXS data, shows six sharp reflections arranged in a perfect hexagon, indicative of uniform homeotropic alignment of a hexagonal COL LC (Figure 3.20d). 96

110 Figure 3.18 Mesogen 1 was sandwiched between two sheets of ITO patterned glass separated by 4μm at 80 C. A) Zero field; B) An AC E-field (10 Hz, 28 V/μm) was applied vertical to the sample under crossed polarizers Applied field (V um -1 ) Temperature ( o C) Figure 3.19 A plot of the switching voltage threshold for driving the reorientation of a parallel sample to homeotropic arrangement as a function of temperature. In addition, with a specially patterned ITO/glass electrode structure allowing application of in-plane and orthogonal electric fields, the LC columns can be reoriented to uniform parallel alignment. Thus, when the mesogen 1 is capillary-filled into a glass LC cell where there are interdigitated finger electrodes (electrode gap 10 m), and a 2.0 Hz square-wave AC E field of ±15 V/µm is applied while the sample is cooled from the isotropic into COL LC phase, POM imaging between crossed polarizer and analyzer demonstrates that the areas between the ITO electrodes show a maximum in transmission when the stripes are oriented at 45 to the polarizers (Figure 3.20b), and exhibit almost no light transmission when parallel or perpendicular to the 97

111 polarizers (Figure 3.21). This demonstrates that clean parallel alignment of the COL LC phase can be achieved by application of in-plane E fields. Figure 3.20 (a and b) POM images (red scale bar represents 100 µm) obtained for LC cells of 1 at 80 C between crossed polarizer and analyzer. Mesogen 1 was filled by capillary action into a sandwich-type glass LC cell with patterned ITO electrodes (the cell gap is 4 µm). (a) An AC square wave electric field (2.0 Hz, ±28 V/µm) was applied to the sample at 80 C. The photomicrograph shows the edge of the electrode area. Domains of unoriented parallel alignment can be seen on the left, off the electrodes with no applied field, and on the right homeotropic alignment over the ITO-coated area. (b) A 2.0 Hz AC square wave in plane field (±15 V/ m) was applied to the sample to provide clean parallel alignment. One 98

112 substrate of the cell was patterned with ITO electrodes with a 10 µm pitch, to provide the in-plane field in stripes between the electrodes (all adjacent electrodes are driven with opposite sign of E. The field was applied while cooling the sample from Iso to Col h. The insert shows a detail of the aligned sample, showing bright stripes of parallel-aligned Col h phase. (c and d) 2D-SAXS patterns obtained from an area off the ITO electrodes (c) and over the ITO electrodes (d) from an LC cell (cell gap 6.4 µm) fabricated using thin (80 µm) glass plates each coated with an ITO square. The sample was heated to 80 C and driven by a vertical E field (±20 V/ m) 10 days before the XRD data was collected. The homeotropically aligned sample (d) shows point-like reflections arranged in a hexagonal lattice. The lattice is indicated by lines added to the image after data collection to guide the eye. (e and f) Schematic illustration of the different types of supramolecular arrangements in (c) and (d) with the incident X-ray beam (indicated by red arrow). Figure 3.21 A square 2.0 Hz E field (15 V/ m) was site-selectively applied to the sample 1 with a 10 m gap of ITO electrodes from a horizontal direction relative to the glass substrates while it was cooling from Iso to Col h. Compared with Figure 3.20b, the magnified (insert) indicates much darker stripes, which were driven by a horizontal E-field (the direction is shown in double red arrow) between two ITO electrodes with a 10 m gap and oriented parallel to the analyzer. 99

113 Based upon the observed reorientation from parallel to homeotropic alignment upon application of an electric field normal to the surfaces, and lack of any suggestion that the phase is ferroelectric (polarization reversal current cannot be detected (Figure 3.22), we propose that the electro-optic switching is dielectric in nature, and the COL phase of discotic mesogen 1 exhibits positive low frequency dielectric anisotropy ( ε) (the reorientation speed was too slow ( 1 min) to allow reliable measurement of the switching time vs. applied field strength). While positive ε is well-known for COL phases, typically ester groups connect flexible chains to a rigid aromatic core in such mesogens, and re-orientation of the ester dipoles in the field is thought to be responsible for the large dielectric constant parallel to the columnar axis. 40 The COL phase formed from a novel discotic mesogen with a helicene core, designed specifically to produce a large dipole normal to the disc, also shows positive dielectric anisotropy. 28b For the mesogen 1, we propose that dipolar reorientation in the triazole-oligothiophene chains is responsible for the observed positive ε. This strongly suggests that, on average, the dipoles of the thiophene units and triazole in each chain are predominantly oriented normal to the long axis of the pentacyclic aromatic chain since the oligomers are mainly oriented parallel to the HBC cores as inferred from XRD. 100